# Local Volume dwarf KK242: radial velocity, SF region, and metallicity

S.A. Pustilnik,1 A.L. Tepliakova,1 Y.A. Perepelitsyna,1 A.Y. Kniazev,2,3,4,1
L.N. Makarova,1 A.N. Burenkov,1 S.S. Kotov,1 E.A. Malygin1
1 Special Astrophysical Observatory of RAS, Nizhnij Arkhyz, Karachai-Circassia
369167, Russia
2 South African Astronomical Observatory, PO Box 9, 7935 Observatory, Cape
Town, South Africa
3 Southern African Large Telescope Foundation, PO Box 9, 7935 Observatory,
Cape Town, South Africa
4 Sternberg Astronomical Institute, Lomonosov Moscow State University,
Universitetskij Pr. 13, Moscow 119992, Russia E-mail<EMAIL_ADDRESS>(SAP)

(Accepted 2022 Spetember 9. Received 2022 September 5; in original form 2022
June 21)

###### Abstract

KK242 is a LV dwarf of transition type residing in the void environment. Koda
et al. present clear indications on its connection with Scd galaxy NGC6503.
This implies the distance to KK242 of $\sim$6.3 Mpc and its MB = –10.5 mag.
Its radial velocity, known from the Effelsberg radio telescope Hi
observations, reveals, however, the difference with that of NGC6503, $\Delta$V
$\sim$ 400 km s-1. If real, this fact implies the substantial constraints on
its origin. To clear-up the issue of KK242 radial velocity, we obtained with
the SAO 6-m telescope spectra of its faint star-forming (SF) complex.
H$\alpha$ and H$\beta$ emission is detected in two adjacent compact regions,
the southern and northern, separated by $\sim$2 arcsec ($\sim$60 pc). Their
mean radial velocity is Vhel = –66 km s-1, $\sim$100 km s-1 lower than that of
NGC6503. We use the HST Legacy Archive images and photometry of individual
stars from the Extragalactic Distance Database, available for KK242, to
identify in the SF complex the exciting hot stars, the probable BHeB and RHeB
stars and a supernova remnant. We address, based on the possible range of its
gas metallicity, the probable evolutionary paths of KK242. Using package
Cloudy and parameters of the exciting B0V stars, we conclude that the observed
flux ratio of [Sii] doublet to H$\alpha$ is consistent with the value of
12+log(O/H) $\sim$7.35 $\pm$ 0.18 dex, expected for a stripped void dIrr
galaxy.

###### keywords:

galaxies: abundances – galaxies: dwarf – galaxies: evolution – galaxies:
photometry – cosmology: large-scale structure of Universe

††pagerange: Local Volume dwarf KK242: radial velocity, SF region, and
metallicity–8††pubyear: 2022

##  1 Introduction

A low surface brightness (LSB) dwarf galaxy of ’transition type’ (dTr) KK242
was first discovered by Karachentseva & Karachentsev (1998) as a potential
companion of Scd galaxy NGC6503. Huchtmeier et al. (2000) observed KK242 in
the 21-cm Hi-line with the Effelsberg 100-m radio telescope and found an
emission line at the radial velocity of 426 km s-1. The latter differs from
the radial velocity of NGC6503 by $\sim$ 400 km s-1 that looks quite unusual.
Later, Koda et al. (2015) rediscovered this galaxy in the deep images of their
survey for LSB dwarfs around spiral galaxies. They presented images of KK242
(their name NGC6503-d1) in various filters, including H$\alpha$. Besides, they
resolved in the broad-band images about three hundred the brightest stars of
KK242. From the analysis of the colour-magnitude diagram (CMD), they derived
the TRGB-based (Tip of Red Giant Branch) distance estimate consistent with
that known for NGC6503.

Both galaxies fall to the region of a nearby void occupying a part of the
Local Volume (hereafter LV, see for more detail Section 4.2). In the framework
of the ongoing project aimed in studying various subsamples of void galaxies
in the Nearby Void Galaxy (NVG) catalog (see Pustilnik et al., 2019, 2020,
2021), we conduct, in particular, their spectral observations to derive their
gas metallicities and/or improve the accuracy of radial velocities. As said
above, Koda et al. (2015) show that there is a compelling evidence of KK242 to
be a companion of NGC6503. However, its known estimate of the radial velocity,
based on a single Hi observation with the Effelsberg 100-m radio telescope, is
too much deviating from that of the host Scd galaxy. This may ’provoke’
various ’exotic’ scenarios of the origin of this pair. This was the primary
motivation to clear up the issue of KK242 radial velocity.

For this end, we obtained the spectrum of the only known faint complex of
H$\alpha$ emission in KK242 detected in papers of Koda et al. (2015) and
Kaisin & Karachentsev (2019). The derived here radial velocity of –66 km s-1
differs drastically from that measured via the single dish 21-cm Hi line
emission. Recently Karachentsev et al. (2022) used our H$\alpha$ radial
velocity to search for the possible Hi 21-cm line emission based on the VLA
D-configuration data cube for KK242. They detected the very faint Hi line at
the position of KK242 with the radial velocity of V(HI) = –80 km s-1, which is
consistent within uncertainties with V(H$\alpha$) obtained in this work.

The estimate of the radial velocity of KK242 via its H$\alpha$ line was our
primary task. Fortunately, thanks to the appropriate seeing during these
observations and a suitable position angle of the long slit, we also got the
interesting by-product results related to the substructure of this emission
region and its individual components. Coupled with the Hubble Space Telescope
(HST) Advanced Camera for Surveys (ACS) images from the Hubble Legacy Archive
(HLA) and with the photometry of individual stars available in the
Extragalactic Distance Database (EDD) (Anand et al., 2021, and references
therein), this enables us to discuss this star-forming complex in a more
detail.

The galaxy KK242 is also interesting for a deeper insight as one of a few
known dTr objects in the void environment. The great majority of the 30 known
dTrs within the distances of 5 Mpc (Karachentsev et al., 2014) are related to
a more typical environment like the Local Group and similar nearby groups.
Such a connection can be related to the origin of this type dwarfs. Only two
of these 30 dTr, UGC1703 and KK258 reside within the nearby voids described in
Pustilnik et al. (2019). Two more dTrs, KKs03 and DDO210, are well isolated,
despite the latter is situated close to the border of the Local Group.

In this context, KK242 as a representative of the small minority of the void
galaxy population, might display various deviations from other known dwarfs of
this rare type. Besides, it is interesting to study the evolutionary path of
such an unusual dwarf in the void-type global environment. The gas
metallicity, if it could be estimated, is one of the important parameters used
for the comparison of the observed properties of KK242 with those expected in
the variety of possible evolutionary scenarios. Moreover, the issue of the
massive star formation in such an atypically low-gas-density dwarf is crucial
to address since this case represents even a more extreme gas environment than
in the late-type gas-normal LSB dwarfs.

The rest of the paper is arranged as follows. In Sec. 2, the spectral
observations and data reduction are outlined and the used archive HST images
are described. In Sec. 3, the results of the analysis of the BTA spectra along
with the identification of the related objects at the HST image are presented.
In Sec. 4, we discuss the properties of the studied SH region in KK242, the
range of metallicity and the global environment of KK242, and in Sec. 5, we
summarize our results and conclude. The linear scale at the adopted distance
to KK242 (6.36 Mpc) is 30.9 pc in 1 arcsec.

##  2 Observations and data processing

Table 1: Journal of BTA observations of KK242 Date | Grism | Expos. | $\beta$ | Air mass
---|---|---|---|---
|  | time, s | arcsec |
2020.11.11 | VPHG1200R | 3$\times$900 | 1.3 | 1.30
2021.11.05 | VPHG1200@540 | 4$\times$1200 | 1.1 | 1.34
2022.07.29 | VPHG1200R | 8$\times$900 | 1.3 | 1.25

Figure 1: Slit positions (PA = –15°) overlaid on the BTA R-band (left panel)
and the HST F606W filter (ProjID 15922) (right panel) images. N is up, E is to
the left. Thin lines show SCORPIO-1 slit (width = 1.2 arcsec), thick lines -
SCORPIO-2 slit (width = 1.54 arcsec). A ’nebula’ in the HST image, with the
apparent diameter of $\sim$0.5 arcsec, is at $\sim$ 3.3 arcsec to the South
from the brightest star of the frame (No. 1). Both objects are visible on the
BTA 2d spectrum in Fig. 2 (top panel). Arrows show the brightest stars within
the BTA slit. Their parameters are given in Table 3. The scales, 30 and 1
arcsec, for left and right panels, respectively, are shown by horizontal bars.
Circle in the HST image shows the FWHM = 1.3 arcsec seeing for the SCORPIO-1
spectrum.

We obtained three optical spectra of KK242. The first spectrum of KK242 was
obtained with the BTA multimode instrument SCORPIO-1 (Afanasiev & Moiseev,
2005) during the night 2020 November 11, under photometric conditions (see
Table 1). The long slit with the width of 1.2 arcsec and the scale along the
slit of 0.36 arcsec pixel-1 (after binning by 2) was positioned on the
brightest R-band source along the elongation of H$\alpha$ emission,
corresponding to PA = –15°(see the left-hand panel of Fig. 1). For a more
detailed description of the slit position relative to the emission-line
regions, see Sect. 4.1. The grism VPHG1200R with the 2K$\times$2K CCD detector
E2V 42-40 (13.5$\times$13.5 $\mu$m pixel) provided the spectrum coverage of
5700–7500 Å with the FWHM $\sim$ 5.0 Å.

The second spectrum of KK242 was obtained with the next generation BTA
multimode instrument SCORPIO-2 (Afanasiev & Moiseev, 2011) during the night
2021 November 5, under photometric conditions (see Table 1). We aimed to pick
up in this spectrum all the light collected in the first observation.
Therefore, accounting for the smaller seeing on this night ($\sim$1.1 versus
1.3 arcsec), we select of the two possible slit width options, 1.0 and 1.54
arcsec, the wider one. This should allow us to directly compare the results
for both spectra. The long slit with the width of 1.54 arcsec and the scale
along the slit of 0.40 arcsec pixel-1 (after binning by 2) was positioned
similar to that in the first observations with PA = –15°. The grism
VPHG1200@540 with the 4K$\times$2K CCD detector E2V 261-84 (15$\times$15
$\mu$m pixel) provided the spectrum coverage of 3650–7250 Å with the FWHM
$\sim$6.0 Å.

The third spectrum of KK242 was obtained with SCORPIO-1 and grism VPHG1200R
during the night 2022 July 29, with the similar set-up as for the first
observation. The long slit for this observation was positioned exactly as for
the first time. Since the seeing also was close to that of the first
observation, the main difference was the total integration time: 7200 sec in
July 2022 versus 2700 sec in November 2020. The main goal of the latter
observation was to improve the S-to-N ratio for [Sii]$\lambda\lambda$6716,6731
doublet in the resulting average spectrum, since from the previous data its
uncertainty was too high to come to more or less confident conclusion on the
KK242 gas metallicity.

The main procedures of data reduction are described in Pustilnik et al.
(2016). Here we briefly outline them. Our standard pipeline with the use of
IRAF111IRAF: the Image Reduction and Analysis Facility is distributed by the
National Optical Astronomy Observatory, which is operated by the Association
of Universities for Research in Astronomy, Inc. (AURA) under cooperative
agreement with the National Science Foundation (NSF). and MIDAS222MIDAS is an
acronym for the European Southern Observatory package – Munich Image Data
Analysis System.  was applied for the reduction of long-slit spectra. It
includes the following steps: removal of cosmic ray hits, bias subtraction,
flat-field correction, wavelength calibration, night-sky background
subtraction. Spectrophotometric standard stars observed during these nights,
were used to obtain spectra in the absolute flux scale.

In the resulting 2d spectra of all observations of KK242, two distinct
H$\alpha$ knots are seen with centres separated along the slit by $\sim$2
arcsec. In Fig. 3 (top panel), we show a part of the 2d spectrum for the first
night. The Southern knot is about twice brighter in H$\alpha$, and has a much
weaker underlying continuum in comparison to that of the N knot. The 1d
spectra for the S and N knots were extracted, summing up 5 or 6 (for different
detectors) and respectively 6 or 7 pixels along the slit ($\sim$ 2.1 and 2.5
arcsec, respectively), without weights, centred on each of two maxima of the
H$\alpha$ line signal. We have no opportunity to compare directly our observed
H$\alpha$ knots with those separated by Koda et al. (2015) on their Subaru
telescope H$\alpha$ image of KK242 with the seeing of 0.8 arcsec. However,
their small angular size on the 2d spectrum indicates that their observed
extent is mainly due to the seeing.

Deep images of KK242 obtained with the HST/ACS in 2019 in the framework of the
SNAP program ’Every Known Nearby Galaxy’ (Prop. 15922, PI R.B. Tully) are
available in the HLA333https://archive.stsci.edu. Stellar photometry of the
resolved stars at the KK242 ACS images is available at the Extragalactic
Distance Database (EDD; Anand et al., 2021). They provide us with the suitable
data to get a deeper insight in the star-forming complex covered by the BTA
long slit.

We used both F606W and F814W HST/ACS images of this program to identify
objects related to the H$\alpha$ emission in our spectrum and to estimate
their available physical parameters such as luminosity, size, colour. Close in
the position to the S knot, in both HST images there is a well resolved almost
round nebulosity with the diameter of $\sim$ 0.5 arcsec (or $\sim$15 pc). No
any reliable star-like counterpart is visible within the nebulosity extent. On
the other hand, a more careful analysis (see Section 3.2) of this HST image
reveals a blue star at $\sim$0.7 arcsec to the North (No. 5 in Fig. 1, right),
which could ionize the surrounding gas and, thus, to also contribute to the
H$\alpha$ emission in this region.

For the Northern H$\alpha$ knot, the situation also appears to be complicated.
We discuss this issue in Section 3.2. The resulting 1d spectra are shown in
the middle and bottom panels of Fig. 2.

Figure 2: BTA spectra for two H$\alpha$ knots in KK242. Top panel: extract of
the 2D spectrum in the range 6540–6580 Å. North is up. The distance along the
slit between the peaks of two H$\alpha$ knots is about $\sim$ 2 arcsec. Peak
of H$\alpha$ emission for the N knot is shifted to the S by $\sim$0.7 arcsec
relative to the position of the continuum peak. See Sec. 4.1. Middle panel:
Average of the three 1d spectra of the N knot with the ’strong’ continuum and
H$\alpha$ flux twice fainter than for the S knot. For the ’undetected’ [Sii]
doublet, we show also the best two-gaussian fit in Fig. 3 (left), which is
used to estimate the flux in this doublet. See details in the text. Bottom
panel: The similar average 1D spectrum of the S knot with the weak continuum,
stronger H$\alpha$ and [Sii].

Figure 3: Two-gauss fitting of the region of [Sii]$\lambda\lambda$6716,6731
doublet in KK242. Dotted lines show the observed signal. Solid lines show the
fitting. See comments on the gauss fitting in the text. Left-hand panel: The N
knot. Right-hand panel: The S knot.

Figure 4: BTA spectra of 2021.11.05 of KK242 for the N and S knots in the range from H$\beta$ to H$\alpha$ and [Sii]$\lambda\lambda$6716,6731. Top panel: The N knot including part of continuum of star No. 1 (the extracted range along the slit of $\Delta$X = [–0.3,–1.5 arcsec] in Fig. 6. Bottom panel: The S region including continuum of the nebula (the extracted range of $\Delta$X = [–1.5,–3.5 arcsec] in Fig. 6.  Table 2: Average (top) and separate nights (bottom) parameters of S and N knots in KK242. Line fluxes in 10-17 erg cm-2 s-1; FWHM in Å.  | S knot (mean) | N knot (mean)
---|---|---
$\lambda_{0}$(Å) Ion | F($\lambda$) | FWHM$\dagger$ | F($\lambda$) | FWHM$\dagger$
6563 H$\alpha$ | 2.98$\pm$0.05 | 5.8$\pm$0.1 | 1.63$\pm$0.07 | 5.1$\pm$0.1
6717 [S ii] | 0.44$\pm$0.11 | 7.9$\pm$0.7 | 0.11$\pm$0.15 | 5.1
6731 [S ii] | 0.23$\pm$0.09 | 6.0$\pm$1.0 | 0.07$\pm$0.13 | 5.1
EW(H$\alpha$) Å | 33.3$\pm$0.7 | 5.5$\pm$0.3
2021.11.05 | S knot | N knot
$\lambda_{0}$(Å) Ion | F($\lambda$) | FWHM | F($\lambda$) | FWHM
4861 H$\beta$ | 1.47$\pm$0.22 | 6.5$\pm$0.9 | 0.44$\pm$0.11 | 8.3$\pm$1.9
5007 [O iii] |  $<$0.30 (2$\sigma)$ | … |  $<$0.20 (2$\sigma)$ | …
6563 H$\alpha$ | 3.62$\pm$0.15 | 5.9$\pm$0.1 | 1.74$\pm$0.11 | 6.2$\pm$0.2
Rad. vel. | –65$\pm$10 km s-1 | –44$\pm$14 km s-1
2020.11.11 | S knot | N knot
Rad. vel. | –80$\pm$10 km s-1 | –59$\pm$14 km s-1
2022.07.29 | S knot | N knot
Rad. vel. | –87$\pm$10 km s-1 | –61$\pm$14 km s-1
Notes. a For S knot, the average FWHMs for all three lines are taken on two
observations with SCORPIO-1 as having the better spectral resolution. For N
knot, the similar average only for the H$\alpha$ line is adopted. For the low-
signal [S ii] lines, the two-gauss fitting was performed with the FWHM equal
to that of H$\alpha$.

Figure 5: H$\alpha$ line and the adjacent continuum emission distribution
along the slit in KK242 spectrum on 2021.11.05. The position of the brightest
continuum peak (X = 0) corresponds to position of Star No.1 at the HST image
in Fig. 1 (right panel). Top panel: Net H$\alpha$ (red), sum of the adjacent
continuum and H$\alpha$ (black), the adjacent continuum (green). The secondary
’peak’ of continuum is at $\sim$3 arcsec to the South, is close to the
positions of the Nebula and star No. 5 and looks to be extended further to the
South. Bottom panel: Same for the region of H$\beta$ emission.

##  3 Results

###  3.1 Emission line parameters of KK242

The results of emission line measurements and analysis for the average spectra
of all three nights are presented in Table 2. For the S and N emission-line
’knots’, we present the absolute fluxes (in units of 10-17 erg cm-2 s-1) for
H$\alpha$ and [Sii] doublet and their widths (FWHM), as well as the equivalent
widths of H$\alpha$ and the measured heliocentric radial velocities with their
uncertainties.

We notice that the measured line widths in the spectra of the S knot are
somewhat broaden relative to the instrumental FWHM $\sim$ 5.1 Å, as measured
for the H$\alpha$ line in the N knot. While for the lines of the [Sii]
doublet, the S-to-N is lower, there is also a hint on their broadening in the
S knot. The spectral resolution for the spectrum on 2021 November 5, is worse
($\sim$6.2 Å as indicated by the the FWHM of H$\alpha$ in the N knot). Also,
for the S knot, this parameter is of the lower accuracy. Hence, we use for the
estimate of the broadening of H$\alpha$ in the S knot only the data from the
first and third observations conducted with SCORPIO-1 with the same set-up.
This broadening from 5.1 to 5.8$\pm$0.1 Å implies that the intrinsic value of
FWHM $\sim$ 2.8$\pm$0.2 Å, or the respective velocity width of $\sim$
128$\pm$9 km s-1.

The radial velocities measured on H$\alpha$ for the S and N knots, averaged on
the three independent datasets, are respectively, –77 $\pm$ 10 km s-1 and –55
$\pm$ 10 km s-1. The radial velocities for the both knots seem to show the
systematic difference of $\sim$20 km s-1 in all three spectra. We have no at
the moment additional arguments to treat one of them to be a more
representative of the galaxy radial velocity. Therefore, we adopt their
average of –66 $\pm$ 10 km s-1 as the robust estimate of KK242 radial velocity
in the line H$\alpha$.

The line [Nii]$\lambda$6584 is not detected. The upper limit on its flux
F($\lambda$) in the S knot, adopted as 2$\sigma$(noise) in the adjacent
continuum multiplied by the instrumental FWHM = 5.0 Å, is 0.2 $\times$ 10-17
erg cm-2 s-1. This is a factor of three smaller than the flux of the [Sii]
doublet.

The relative flux of [Sii] doublet to that of H$\alpha$ is a sensitive
parameter for checking the possible range of gas metallicity in KK242. For the
S knot, the contribution of the emission from the nebula, a probable SN
remnant, can substantially exceed the expected contribution from an Hii region
excited by a nearby hot massive star (see discussion in Sect. 4.1). For the N
region, we have an unambiguous case, in which only the nearby hot stars
provide the ionizing radiation, so that [Sii] lines should reflect the
abundance and physical conditions characteristic of normal Hii regions.

The comparison of the three individual spectra of the N knot indicates that
the strength of the [Sii] doublet relative to that of H$\alpha$ varies by a
factor of 2–3. This means that these faint lines are in fact at the level of
about one $\sigma$ of the noise or so. Therefore, discussion of [Sii] doublet
in the individual spectra is irrelevant. To derive a more reliable estimates
of the strength of the [Sii] doublet, we obtained the weighted mean of all
three individual spectra with the weights adopted as 1/$\sigma_{\rm
noise}^{2}$. Here the noise was estimated on the wavelength range between
H$\alpha$ and [Sii] doublet. As one can see in Figure 2 (middle) and Figure 3
(left), the signal in the individual lines of the doublet in the N knot is
comparable to the noise spikes.

We employ the next approach to get the best approximation of the real line
fluxes. We use all a priori information on the line positions and widths based
on the measured parameters of the H$\alpha$ line. We also adopt the low-
density case and the related flux ratio I(6716)I(6731) = 1.4. With these
parameters fixed, we vary the amplitude of the main peak to get the lowest
value of the residuals. The resulting line fluxes are shown in the top part of
Table 2. For further discussion, we use for the N knot the parameter of flux
ratio [Sii]/H$\alpha$ = 0.18/1.63 = 0.11, with the conditional uncertainty of
50%, that is 0.11 $\pm$ 0.055. For the S knot, the respective value of
[Sii]/H$\alpha$ is 0.67/2.98 = 0.225 $\pm$ 0.073. We use this parameter in
Section 4.3.

In Fig. 4, we present a wider range spectra of both the N and S knots of
KK242. They show the absence of the [Oiii]$\lambda$5007 that is consistent
with the very low excitation in the discussed Hii regions.

For discussion of the possible interpretation of the visible nebular emission,
it is useful to examine the H$\alpha$ flux distribution along the slit
relative to the northern peak of continuum related to the brightest red star
at the HST image, star No. 1 (see the right-hand panel of Fig. 1). In the top
panel of Fig. 5, we show the flux distribution along the slit for
H$\alpha$-line emission (red), for the adjacent continuum (green) and for
their sum (black). To tie the relative positions of the line emission to the
position of the ’bright’ red star No. 1 in Fig. 1, we count X-axis coordinate
in Fig. 5 from the position of the highest peak of continuum which corresponds
to this star. Negative values of X-axis correspond to the direction
approximately to the south.

One can see that H$\alpha$ emission has a two-peak distribution, both shifted
to the S relative to star No. 1. A two-Gaussian fit to the flux distribution
allows us to get their positions as X(S) = –2.5 arcsec and X(N) = –0.7 arcsec.

Finally, the total flux of H$\alpha$ is adopted as a sum of the nominal fluxes
in S and N knots according to the data on 2021.11.05 as obtained with the
better seeing and a wider slit (bottom of Table 2), namely F(H$\alpha$) =
(5.36 $\pm$ 0.18) $\times$ 10-17 erg cm-2 s-1. We expect that due to the loss
of a fraction of H$\alpha$ emission falling outside the slit, the real flux in
this region can be a factor of 1.5–2 larger. We address the comparison of this
parameter with earlier data in Section 4.1.

###  3.2 HST objects in the SF region

In Table 3, we present positions and parameters of the brightest 8 stars and
of one nebula which fall within the central area of the BTA long slit. They
all can contribute to the visible continuum and the line emission. All these
objects are marked in Fig. 1 (right). This should help to identify them in the
course of the further discussion. Their HST/ACS pixel coordinates and visible
V and I magnitudes are taken from the table KK242.phot.WEB at the EDD
site444https://edd.ifa.hawaii.edu/get$\\_$cmd.php?pgc=4689184, where
parameters of all measured in KK242 636 stars are presented.

Their world coordinates are taken from the available HST fits images of this
region. For the nebula, we obtained our own aperture photometry with the
radius of 0.25 arcsec, which encompasses the whole object’s light. We linked
its instrumental magnitudes with those derived for star No. 1. From their
difference, we derived the V and I magnitudes of the nebula, presented in
Table 3. We further discuss objects identified at the HST images in Sect. 4.1.

Table 3: HST based parameters of objects in the studied SF region of KK242 Object | x | y | Coordinates (J2000) | Vmag | Imag | MV,0 | (V–I)0 | Approx.
---|---|---|---|---|---|---|---|---
| px | px | relative to 17:52, +70:08 |  |  |  |  | classif.
No. 1 | 2128.7 | 3087.6 | 47.084, 11.25 | 22.14$\pm$.01 | 20.60$\pm$.01 | –6.98 | +1.51$\pm$.01 | RHeB
No. 2 | 2120.5 | 3085.6 | 47.101, 10.87 | 24.82$\pm$.03 | 24.82$\pm$.05 | –4.30 | –0.02$\pm$.06 | BHeB
No. 3 | 2107.4 | 3082.2 | 47.128, 10.23 | 25.79$\pm$.05 | 26.15$\pm$.12 | –3.23 | –0.37$\pm$.13 | B0V
No. 4 | 2095.9 | 3082.9 | 47.177, 09.70 | 26.23$\pm$.07 | 26.52$\pm$.16 | –2.79 | –0.32$\pm$.17 | B0V-B1V
No. 5 | 2075.0 | 3077.9 | 47.211, 08.67 | 25.69$\pm$.05 | 25.88$\pm$.10 | –3.33 | –0.21$\pm$.11 | B0V-B1V
No. 6 | 2099.8 | 3096.4 | 47.285, 10.15 | 25.63$\pm$.05 | 26.10$\pm$.12 | –3.39 | –0.49$\pm$.14 | O9V
No. 7 | 2092.7 | 3091.9 | 47.271, 09.74 | 24.71$\pm$.03 | 24.62$\pm$.04 | –4.41 | +0.07$\pm$.05 | BHeB
No. 8 | 2070.2 | 3087.0 | 47.315, 08.60 | 26.48$\pm$.07 | 25.09$\pm$.06 | –2.54 | +1.37$\pm$.08 | RGB?
Nebula | 2062: | 3073: | 47.231, 07.94 | 25.12$\pm$.07 | 23.44$\pm$.03 | –4.00 | +1.66$\pm$.08 | SNR
Notes. 1\. For Nebula: coordinates of the centre and our aperture photometry
with the radius of 5 pixels (0.25 arcsec).
2\. Only stars are shown falling within the region of H$\alpha$ emission with
V $<$ 27.0 mag (M${}_{\rm V,0}\lesssim$ –2.0).
3\. E(V–I) is adopted 0.02 mag.

##  4 Discussion

Table 4: Properties of KK242 Property | Value | Refs
---|---|---
RA (J2000) | 17 52 48.4 | 4
Dec (J2000) | +70 08 14 | 4
Rad. velocity (H$\alpha$), km s-1 | –66$\pm$10 | 1
Rad. velocity (Hi), km s-1 | –80$\pm$10 | 2
$D_{\rm adopt}$ | 6.36$\pm$.20 | 3
Distance mod. (mag) | 29.02 | 2
B${}_{\rm tot}^{1}$ (mag) | 18.64 | 4
AB (mag) | 0.14 | 5
$M_{\rm B}$(mag) | –10.5 | 2
$\mu_{\rm 0,B}$ (mag arcsec2) | 25.5 | 4
S(Hi) (Jy km s-1) |  $\lesssim$0.1 | 2
M(Hi) (in 106 M⊙) | 1$\pm$0.5 | 2
M(*) (in 106 M⊙) | 6.0 | 4
Notes. 1\. This work (weighted average of both knots). 2. Karachentsev et al.
(2022). 3\. Adopted as a mean of distances to KK242 and NGC6503 from
Karachentsev et al. (2022). 4\. Koda et al. (2015), M(*) scaled to the
distance of 6.36 Mpc. 5\. The Milky Way extinction after Schlafly & Finkbeiner
(2011).

###  4.1 Morphology of the SF region and related issues

The long-slit position for spectroscopy of the Hii region in KK242 was
determined based on the published H$\alpha$ images available at that time
(Koda et al., 2015). The slit PA = –15° was also chosen to put the slit closer
to the visible elongation of the Hii region in order to increase the chances
to catch a knot with a higher excitation. As the analysis of the HST image in
Fig. 1 (right) reveals, the slit properly covers all hot stars except the
bluest one, no. 6. For this object, accounting for the seeing of 1.1–1.3
arcsec, we probably lose up to a half of the star and the related Hii region
flux relative to the other discussed stars.

Besides, as described at the end of Sect. 3.1, the nominal measured flux of
H$\alpha$ in this region is $\sim(0.54\pm 0.02)\times 10^{-16}$ erg cm-2 s-1,
with the possible upwards correction by a factor of 1.5–2 due to the loss on
the slit. There are two independent estimates of this parameter (obtained from
H$\alpha$ imaging, in units of 10-16 erg cm-2 s-1) in papers by Koda et al.
(2015) (2.3 with the uncertainty of factor of two) and Karachentsev et al.
(2022) (4.7$\pm$2.5). Taking into account their large uncertainties and our
upward correction of the nominal flux of H$\alpha$, we suggest that the real
flux of H$\alpha$ in this region is of $\sim$10-16 erg cm-2 s-1.

Our task is to connect the visible H$\alpha$ emission on the BTA long-slit
spectra with the exciting hot stars visible in this region at the HST image.
This allows us to better understand probable parameters of the SF episode in
this region. Besides, this information will be helpful in the attempt of
modelling the observed nebular emission (e.g. with Cloudy package, see the
Appendix).

As the derived (V–I)0 colours of stars in Table 3 indicate, the two brightest
stars, no.1 and no.2 are not hot and therefore have no relation to the
observed nebular emission. No.1 is likely a Red Helium Burning (RHeB), while
no.2 and 7 with the colour (V–I)0 $\sim$ 0.0 mag are likely Blue Helium
Burning (BHeB) stars (McQuinn et al., 2011, 2012). Both types are products of
the late evolution with the He-core burning of stars with the intermediate
masses of 2–15 M⊙. The measured absolute magnitudes of MV = –7.0 mag for star
no. 1 and –4.3 and –4.4 mag for no.2 and 7, evidence to the extended episode
of SF in this region, with the duration of at least of $\sim$100 Myr (McQuinn
et al., 2011).

Stars no.3, 4, 5 and 6 all are rather blue and luminous, seemingly
representing the main-sequence late O-type and/or early B-type stars. We use
the sample of O and B stars from the Wing of the Small Magellanic Cloud (SMC)
of Ramachandran et al. (2019) for which these authors present the observed and
model physical parameters. Since this large sample of OB stars have the
nearest metallicity and thus are the best proxies to KK242 massive stars, we
use their MV as a comparison for our blue luminous stars. As one can see, the
SMC OB stars show the substantial scatter in their MV. Accounting for this
information, the blue stars found in the KK242 SF complex can be assigned to
the range of O9V to B1V. The bluest (hottest) star No. 6 is probably an O9V,
while the remaining, a bit redder stars, are probably of B0V–B1V type.

The positions of blue stars no. 3, 4 and 6 are close to the position of the
brightness peak of the Northern H$\alpha$ knot. Respectively, the position of
the brightness peak of the Southern H$\alpha$ emission is close to the
position of the blue star no. 5. On the other hand, it is also rather close to
the position of the adjacent red nebula, so that it can contribute to the
observed nebular emission of the S region as well.

Since the projected distances between the hot stars no. 3, 4 and 6 are of
$\sim$20–30 pc, for the gas density N $\gtrsim$ 10 cm-3, their Strömgren radii
should be smaller than their mutual distances, so it is unlikely that they
form a common Hii region. However, the separate Hii regions around these stars
can contribute to the total observed nebular emission in the northern
H$\alpha$ knot due to the insufficient angular resolution along the slit. A
similar situation takes place for the southern H$\alpha$ emission. Here, the
hot star no. 5 is a clear candidate for the line emission from the related Hii
region. However, the adjacent nebula, a probable SN remnant, can make a major
contribution.

Indeed, as one can see from Table 2, the H$\alpha$ line flux is a factor of
two larger in the southern knot relative to that in the northern knot. Taking
into account that in the N region, we have three hot massive stars (no. 3, 4,
6) with parameters close to those of star no.5 in the S knot, it is reasonable
to assume that the H$\alpha$ luminosity of Hii region excited by star no.5 is
lower (about factor of 2–3) than the respective luminosity for the three Hii
regions comprising the N knot. From this consideration it follows that the
main contribution to the line and continuum emission of the S knot comes from
the nebula, a candidate SN remnant.

This coarse analysis shows that the observed spectra of this SF complex, with
the limited angular resolution typical of the ground-based observations,
represent rather complicated combination. So that their interpretation, even
for a higher S-to-N case, can be not that straightforward. They can hope that
for the exceptional ground-based observational conditions, with a seeing of
$\lesssim$0.5 arcsec and with the appropriate slit width oriented at a proper
direction, one can disentangle the contribution of various Hii regions and
enable obtaining of the strong-line fluxes and the subsequent estimates of the
gas metallicity. More chances for the detection of the strong oxygen lines are
expected for the Hii region around star no. 6, the hottest of all massive
stars in KK242, judging on its V–I colour. Also, the projected distance of
$\sim$0.7 arcsec between star no. 5 and the centre of the nebula should be
sufficient to disentangle emission lines of the two objects.

###  4.2 Environment

The dwarf galaxy KK242, as well as its host spiral galaxy NGC6503, resides
within a nearby void, No. 22 (also named Dra-Cep) in the list of Pustilnik et
al. (2019). Their distance to the nearest luminous galaxy NGC6946 is DNN =
2.25 Mpc. The void is a bit flattened spheroid with the large diameter of 21
Mpc. The centre of the void is at the distance of 13.9 Mpc from the Local
Group, in the direction of RA = 20.4 h, Dec. = +71°. The Dra-Cep void is
situated above the supergalactic plane SGZ=0, being adjacent at SGX $\sim$ +10
to +20 Mpc to the largest nearby void Oph-Sgr-Cap which includes the well
known Local (Tully) Void (see illustration in fig. A5 of Pustilnik et al.
(2019)). Fifty galaxies reside within the void boundaries (Pustilnik et al.,
2019). Of them, 44 are classified as the ’inner’ void galaxies, defined as
those with the distance to their nearest luminous neighbour $D_{\rm NN}\geq$
2.0 Mpc. Both KK242 and NGC6503 are assigned to the ’inner’ void galaxies.

###  4.3 Evolutionary scenarios and gas metallicity

####  4.3.1 KK242 as a void dwarf of transition type

The phenomenon of dTr is not well understood. The assignment of a dwarf to the
transition type is a purely phenomenological. While these dwarfs have in
general the substantial amount of gas, comparable with that in dIrrs, their
current star formation as traced by the H$\alpha$ emission of the related Hii
regions, is (almost) quenched. Five the nearest and seemingly the best studied
examples of dTrs (or dIrr/dSph) are found within the Local Group and reviewed
by Mateo (1988). Skillman et al. (2003) added to them three similar dwarfs in
the Sculptor group and via the analysis of the HST CMDs addressed the nature
of ”transition” dwarfs. They conclude that the examined dTrs are similar on
the gas content to dIrr and are ’found preferentially among the lowest
luminosities and nearer to spiral galaxies. Their appearance thus is caused by
the temporary interrupted star formation. However, the tidal effects of
massive hosts also may play a role’.

Later Weisz et al. (2011), with deep HST data, studied SF histories (SFHs) of
60 dwarfs within 4 Mpc. Of them, 12 are classified as dTrs. One of their
conclusions is that despite the large diversity, the mean SFHs of dIrrs, dTrs
and dSphs are similar over most of cosmic time, with the clearest difference
between the three only during the most recent 1 Gyr. They also conclude: ’In
terms of their environment, SFHs and gas fractions, the majority of the dTrs
appear to be low-mass dIs that simply lack H$\alpha$ emission, similar to the
LG dTr DDO210. However, a handful of dTrs have remarkably low (but detectable)
gas fractions, suggesting that they nearly exhausted their gas supply,
analogous to the LG dTrs such as Phoenix and LGS3.’

As mentioned in Section Introduction, to date, the great majority of the known
dTrs are found in groups and near massive hosts (Karachentsev et al., 2014).
This can be, at least partly, due to the observational selection effects,
since for the faint isolated dwarfs of this type, the determination of their
radial velocity, similar to the case of dEs, is a difficult task.

One can think on the different origin and evolutionary scenarios of dTr
objects, that can be related, in particular, to their global environment. For
the majority of the currently known dTrs, the simplest scenario assumes their
relation to ’normal’ dIrrs with the lowest baryonic mass, in which the
intermittent SF occurs with the duty cycle larger than (a few) tens Myr. For
the alternative scenarios, in particular, for gas-poor dTrs, there are various
options. For example, a normal dIrr progenitor could lose the major part of
its gas due to the close passage near a more massive host (e.g., Di Cintio et
al., 2021). Another option is a long-ago formed dSph/dE which was recently
rejuvenated due to the gas accretion. As far as we aware, the variant of the
metal-enriched gas accretion from the outer parts of a massive host to a dwarf
companion is not yet modelled. That is one can not describe, at which
circumstances this will occur, if at all. On the other hand, in voids, where
gas velocities in filaments are low (Aragon-Calvo & Szalay, 2013), the
unprocessed gas accretion from filaments to small dwarfs can probably work.

Coming back to the properties of KK242, we notice, that on the low gas content
it resembles the minority of dTrs, which ’nearly exhausted their gas supply’,
in difference to the main group of dTrs. Due to the similar SFHs of dIs and
dTrs, with except of the last 1 Gyr (Weisz et al., 2011), one of the
evolutionary scenarios for dTr objects involves the late gas removal and the
related star formation quenching. Therefore, one can expect that the
progenitors of gas-poor dTr objects have been evolving most of their lifetime
similar to dIrrs with the same mass. Then, if the metallicity of the removed
gas was typical of the gas in the whole galaxy, the remaining gas should be
representative of the previous secular evolution. In this scenario, it is
probable that the gas metallicity in dTrs is similar to that in dIs with the
same stellar mass and luminosity.

####  4.3.2 Gas metallicity as expected from the global parameters

The gas metallicity of late-type galaxies in the LV follows the trend
described by the relation of ’O/H versus MB’ from Berg et al. (2012). The
respective linear regression reads as 12+log(O/H) = 6.272 – 0.107 $\times$ MB,
with the rms scatter of log(O/H) of 0.15 dex. It extends over the range of MB
= [–9.0,–19.0]. The great majority of this LV reference sample belongs to
typical groups and their close environs. As shown in Pustilnik et al. (2016,
2021), the late-type dwarfs in the nearby voids have, on average, the reduced
values of log(O/H) by 0.14 dex (or by $\sim$30 percent, with the rms scatter
of 0.18 dex) relative to this reference relation. This finding was interpreted
as an evidence of the slower galaxy evolution in voids. Consistently with this
idea, void galaxies have also the elevated Hi content, on average by 40
percent (Pustilnik, Martin, 2016).

Since KK242 is not a typical late-type dwarf, the above statistical relations
between O/H and blue luminosity or stellar mass, derived in Berg et al.
(2012), may be not directly applicable to it. Those relations are assumed to
reflect the specifics of the secular evolution of disc galaxies in the wide
range of baryonic mass.

It is interesting to compare its gas metallicity with the other dwarfs of the
same blue luminosity. We first derive the expected gas O/H in KK242 if its Hii
region(s) metallicity obeys the above reference relation for the Local Volume
late-type galaxies from Berg et al. (2012). For MB(KK242) = –10.5 mag, its
expected 12+log(O/H) = 7.40 $\pm$ 0.15 dex. Koda et al. (2015) present the
estimate of the total stellar mass for KK242 for their adopted distance of
5.27 Mpc. Accounting for the scaling due to the increased distance by a factor
of 1.2, we adopt it as log(M∗) = 6.78 dex.

We can use this log(M∗) for an alternative estimate of the gas O/H, based on
the similar relation from Berg et al. (2012), namely: 12+log(O/H) = 5.61 +0.29
$\times$ log(M∗), with the rms scatter of $\sigma$ = 0.15 dex. This gives the
value 12+log(O/H) = 7.58$\pm$0.15 dex. Taking the average of the two
independent estimates (7.40 and 7.58), we adopt the expected value of gas O/H
for a typical dIrr with those MB and stellar mass, as 12+log(O/H) =
7.49$\pm$0.10 dex.

If we take into account that the secular evolution of KK242 took place within
a void, then, as mentioned in the beginning of this section, the expected
value of O/H is lower, on average by 0.14 dex, that is, of 12+log(O/H) = 7.35
$\pm$ 0.18 dex.

####  4.3.3 Use of [Sii] doublet to constrain gas O/H

With a lack of information on the strong oxygen lines, our spectral data, on
the first glimpse, cannot be used to derive more or less reliable empirical
estimate of O/H. The only means to probe gas metallicity in KK242 and to
check, whether this is consistent with the above estimate of the expected O/H,
is the relative strength of [Sii] doublet. Its statistical relation with the
parameter 12+log(O/H) can provide us, in principle, with the possible range of
the gas metallicity of KK242 and help in the comparison with the gas
metallicity expected within a particular scenario in the previous section.

In the following discussion of the N knot, we adopt the flux ratio of the
[Sii] doublet and H$\alpha$ as a weighted mean of the three independent
measurements, as shown in the top of Table 2, namely 0.110$\pm$0.055. We adopt
conditionally, for illustration, the error at the level of 50%. From the
formal estimates, it’s probably twice larger. In the further comparison of the
strength of [Sii] doublet to galaxies with known O/H, we need its ratio to the
flux of H$\beta$. We adopt it from the typical flux ratio of H$\alpha$ and
H$\beta$ for the Case B recombination, of $\sim$2.8. The latter is consistent
with the flux ratio of the N component as visible in the intensity cuts along
the slit in Fig. 5 (top and bottom). Then, the respective parameter, called S2
(a ratio of [Sii] doublet flux to that of H$\beta$), is adopted for further to
be equal to 0.31+-0.155. That is the most probable range of S2 is
[0.155,0.465]. The respective value of lg(S2)=–0.509, with the most probable
range of [–0.810,–0.333].

The parameter log(S2) can be used in principle for comparison with the
statistical data compiled by Pilyugin et al. (2012) for various Hii regions in
galaxies with the wide range of O/H. In Fig. 6, we plot the relation between
the parameter log(S2) and 12+log(O/H) for a subsample of 161 data points from
the compilation by Pilyugin et al. (2012) for all Hii regions with 12+log(O/H)
(the direct method) in the range of $\sim$ 7.1 – 8.0 dex. The vertical solid
black and blue dashed lines mark the expected value of gas O/H for the
absolute blue magnitude and stellar mass of KK242 and its $\pm$1 rms corridor
for the case when the gas metallicity (O/H) of KK242 obeys the reference
relation for late-type galaxies from Berg et al. (2012).

As discussed above, for the S region, the H$\alpha$ emission appears to be the
sum of two components. The first is an Hii region excited by the star no. 5, a
probable star B0V-B1V, and the second is the emission from a round red nebula
without a detectable exciting central star. As we argued above, the main
contribution to the emission of this region is due to radiation from the
nebula. The profile of H$\alpha$ line in this region looks broaden by the
amount corresponding to the intrinsic FWHM $\sim$ 128 km s-1. The latter
corresponds to a shell expansion with the characteristic velocity of $\sim$64
km s-1. Therefore, it is very likely that the round nebula is a supernova
remnant (SNR) with an age of less than 1 Myr.

It is well known that the ratio of [Sii]$\lambda\lambda$6716,6731 flux to that
of H$\alpha$ (hereafter, [Sii]/H$\alpha$) is enhanced in the optical spectra
of SNR due to the shock excitation. The often used empirical criterion to
assign the observed emission to the shock-excited is [Sii]/H$\alpha>$ 0.4.
However, Kopsacheili, Zezas & Leonidaki (2020) from the analysis of the shock-
excitation models based on the package MAPPINGS III (Allen et al., 2008) show
that this parameter can be substantially smaller for the subsolar gas
metallicities and for the shock velocities less than 200 km s-1. Therefore,
the observed in the Southern knot ratio [Sii]/H$\alpha\sim$ 0.225, accounting
for a low gas metallicity and a ’small’ shock velocity, is consistent with the
expected in the models. Therefore, for the S region, we can not use this ratio
for comparison with the statistical data for normal Hii regions.

Figure 6: Plot of parameter log(S2) versus 12+log(O/H) for observed and model
Hii regions. Black octagons show points for 161 Hii regions with
12+log(O/H)(Te) $<$ 8.0. They are drawn from the compilation of the literature
data in Pilyugin et al. (2012). Vertical solid black and blue dashed lines
show the expected value of 12+log(O/H) and its probable variance
(7.49$\pm$0.10 dex), corresponding to MB and stellar mass of KK242, in the
case if its O/H follows the reference relation ’O/H versus MB’ for the LV
sample of Berg et al. (2012). Green solid and dashed vertical lines show the
probable O/H for similar galaxies residing in voids (7.35 dex) and its
+1$\sigma$ value (7.53 dex) (Pustilnik et al., 2021). The horizontal solid red
and two blue dashed lines show the nominal value of log(S2) for KK242 Northern
knot and its $\pm$1$\sigma$ range. We also draw the linear regression of
log(S2) on log(O/H) for the above sample of 161 points (solid black) and its
upper envelope (dotted black). Red and green diamonds correspond to the
maximal values of S2 for a given value of O/H, which occur for models with the
values of the ionisation parameter of lg(U) = –4.0 and –3.0, respectively.
They occur for the lowest considered Teff of the exciting stars, of 25–30 kK,
as illustrated in Fig. A1 of Appendix A. The red dotted line shows the linear
approximation of of positions of the red diamonds. We also add 9 points with
the direct values of 12+log(O/H) $\lesssim$7.32 dex (blue squares), published
after 2012 (see references in the text), to illustrate the large scatter of
the parameter S2 for the low values of O/H in comparison to the limited data
from Pilyugin et al. (2012). See text for details and discussion of this
figure.

For the N region, we adopt that the nebular emission, visible on the slit, is
the sum of emission of three normal Hii regions excited by the hot stars no.
3, 4 and 6. In Fig. 6, the nominal value of log(S2)(KK242) = –0.509 for the N
knot is shown by the red horizontal line, while the lines, corresponding to
+1$\sigma$ (–0.333) and –1$\sigma$ (–0.81) are shown by the dashed blue lines.

As well seen in Fig. 6, the nominal value of log(S2)(KK242), being directly
compared to the data points from Pilyugin et al. (2012), corresponds to rather
wide range of 12+log(O/H) $\sim$ 7.8 $\pm$ 0.2 dex. The latter is
significantly larger than the O/H expected for its very low MB and M${*}$.
This apparent inconsistency needs explanations and a discussion.

Since the used data from Pilyugin et al. (2012) is based on Hii regions with
the directly measured O/H, they all have the well detected line [Oiii]4363.
According to the Cloudy models discussed in the Appendix, this case
corresponds to Hii regions with the sufficiently large value of the ionization
parameter, namely to log(U) $\gtrsim$ –3.0. According to the same model grids,
for a given gas metallicity, the parameter S2 can vary substantially for the
range of Teff of the exciting stars of [25, 50] kK and the range of log(U) =
[-4.0,-1.0].

To discuss more general cases, we draw in Fig. 6, in addition to the observed
Hii regions with the direct O/H from Pilyugin et al. (2012), the model-
predicted values of S2 for a wider range of log(U) and Teff, derived with the
Cloudy package in Appendix. Here, green and red diamonds show the maximal
values of log(S2) for nine values of 12+log(O/H) between 7.00 to 8.04 dex for
log(U) = –3.0 and –4.0, respectively. These maximal values of S2 correspond to
the minimal values of Teff from the grid with Teff = 25–50 kK with the step of
5 kK.

Coming back to the N region of KK242, we notice that its nominal value of
log(S2)=-0.51 corresponds to 12+log(O/H)=7.44 dex in the case of this region
is ionized by the lowest Teff (25–30 kK) and the lowest log(U) source ($\sim$
–4). This is well consistent with the expected void dwarfs metallicity
mentioned above of 12+log(O/H) = 7.35 $\pm$ 0.18 dex.

It is worth to noting that the sample of the reference Hii regions from
Pilyugin et al. (2012) does not cover completely the real parametric space in
the plane 12+log(O/H), log(S2) for regions with the directly derived O/H, at
least for the lowest gas metallicities. We added in Fig. 6 ten data points
with the direct 12+log(O/H) $\lesssim$ 7.3 dex (blue squares), appeared in the
literature after 2011 in papers by Izotov, Thuan & Guseva (2012); Izotov et
al. (2018); Izotov, Thuan & Guseva (2019); Skillman et al. (2013), Hirschauer
et al. (2016); Hsyu et al. (2017); Pustilnik et al. (2021). Seven of them fit
well the region defined by the data from Pilyugin et al. (2012) and its
extension to the lower O/H. However, the parameter log(S2) for ’Little Cub’
and regions UGC772-N2 and N3 falls significantly higher than for the remaining
majority. In particular, the region UGC772-N2 has the S2 parameter close to
that of the N knot in KK242 and very low 12+log(O/H) $\sim$ 7.3 dex.

####  4.3.4 Probable parameters of exciting stars in KK242. Similar case of
Pegasus DIG

To complete the discussion on the consistency of the observed parameter S2 in
the Northern H$\alpha$ knot in KK242 with the expected low gas metallicity, of
12+log(O/H) $\lesssim$ 7.49 $\pm$ 0.10 dex, we examine the range of possible
parameters of the hot massive stars illuminating this H$\alpha$ region. We are
interested whether their effective temperatures and the ionising photon fluxes
are consistent with the Cloudy package parameters resulting in the largest
values of S2 for that low gas metallicity (red and green diamonds in Fig. 6).

The observed parameters of the related stars nos. 3, 4 and 6 are summarized in
Table 3 and discussed in Section 4.1. We use the results of modelling of a
large sample of massive OB stars in SMC presented in Ramachandran et al.
(2019). The metallicity of this sample is the closest to that expected for gas
and young stars in KK242. Thanks to the good statistics and the large set of
modelled physical parameters, this allows one to use the average parameters
for a star of the given spectral class as well as to understand the real range
of their scatter.

The rough estimate of the expected parameter log(U) in the considered region
can be derived taking the typical flux of the ionising photons of B0V stars in
the SMC, presented in Ramachandran et al. (2019), as follows: log(Q) =
47.26$\pm$0.3 and the effective temperature of Teff = 30$\pm$1 kK.555We notice
that these parameters differ substantially downwards from the adopted in Table
2.3 in the monograph of Osterbrock & Ferland (2006). The expected gas density
is n${}_{\rm e}\lesssim$ 10 cm-3. This leads to the Strömgren radius of
$\sim$15 pc and the related value of lg(U) = –4.0.

As the examination of the Cloudy grids in Appendix shows, for 12+log(O/H) =
7.35, the largest value of parameter S2 occurs at lg(U) = –4.0, and for the
lowest Teff = 25–30 kK, reaching the value of 0.26. Similarly, for grid with
12+log(O/H) = 7.53, the largest value of parameter S2 occurs at log(U) = –4.0,
reaching the value of 0.37. The nominal value of the observed parameter S2 =
0.31, midway between the values of S2 for the latter the lowest log(U) models,
implies that the respective value of its 12+log(O/H) falls midway between 7.35
and 7.53 dex, that is $\sim$7.44 dex. For this combination, the expected ratio
of F(3727)/F(H$\beta$) = 1.2 – 1.4, F(5007)/F(H$\beta$) $\lesssim$ 0.02. It is
worth to noting that despite the current data on the value of parameter S2
allows us to safely assign the N region to the very low metallicity regime,
the higher S-to-N fluxes for the [Sii] doublet are required to increase the
accuracy of the derived O/H.

It is also interesting to compare the spectra of the N knot in KK242 with that
of the Hii region ’A’ (a brighter of two known) in the nearby dwarf galaxy
Pegasus DIG (DDO 216=UGC 12613). In some papers this galaxy is assigned to dTr
due to its morphology and very low current SFR. Its the adopted modern
distance, derived with TRGB by Jacobs et al. (2009), is of 0.97 Mpc. With the
total blue magnitude Btot =13.22 and MB = –12.43 (HyperLEDA), PegDIG is almost
2 magnitudes more luminous than KK242. The spectrum of this Hii region was
analyzed by Skillman et al. (1997). Its parameter S2 = 0.76$\pm$0.05 is
$\sim$2.5 times larger than the observed in the N knot of KK242
(0.31$\pm$0.15). The lines visible in the DDO216 region ’A’ spectrum in the
range of 3700 – 5300 Å, indicate very soft ionising radiation, with the flux
ratio of [Oiii]$\lambda$5007 and [Oii]$\lambda$3727 (hereafter parameter O32)
less than 0.03 (at the 2$\sigma$ level). As the authors argue, that low
excitation spectrum can be explained only by a sufficiently low effective
temperature of an exciting star, namely, T${}_{\rm eff}\lesssim$ 32.5 kK,
consistent with a star B0V. As our Cloudy grids evidence, this value of S2
indeed emerges for the case of gas metallicity of 12+log(O/H)=7.92 as
estimated by Skillman et al. (1997), for a star with Teff = 30 kK when log(U)
= –4.0. The respective parameter O32 appears of $\sim$0.015, consistent with
the observed upper limit. However, the model prediction for this case for the
relative flux of [Oii]$\lambda$3727 and H$\beta$ appears $\sim$2.0, a factor
of 1.7 lower than the observed one.

####  4.3.5 Alternative value of KK242 gas metallicity and a possible related
scenario

In the previous sections we presented the arguments that in the N knot of
KK242, the measured parameter S2 is well consistent with the value of the gas
O/H, expected from Berg et al. (2012) reference relations for log(O/H) versus
MB and versus M${*}$, and also with the reduced gas metallicity as expected
for a void galaxy. With this low gas metallicity one could think on the
typical late-type galaxy secular evolution and the ’recent’ loss of the main
gas mass and the related drop of the ’normal’ star formation.

Since the real uncertainty of the nominal value of S2 is $\sim$100%, we should
check the variant of interpretation of a twice larger value of S2, that is
0.62, or log(S2) = –0.21. As one can see in Fig. 6, this value of log(S2) is
close to the Cloudy model points for log(U) = –4.0 (red dotted line) at
12+log(O/H) $\sim$ 7.8 dex that is too high for void dIrrs with the luminosity
and mass similar to that of KK242.

What kind of scenario could result at that elevated gas metallicity for the
very low stellar mass and luminosity of KK242? One of the possible variants is
related to the gas inflow to KK242 from the outer parts of the disc of NGC6503
in course of their pericenter passage. We did not find in the literature the
published estimates of the gas metallicity in NGC6503 despite its 2D spectra
in the range 3600–6800 Å were obtained with the integral field unit VIRUS-P
(Blanc et al., 2013). Therefore, we adopt for NGC6503 the expected gas
metallicity, that follows from the relation in Berg et al. (2012) for the
Local Volume late-type galaxies. For its absolute magnitude of MB = –19.1 mag
(HypeLEDA, averaged on several sources), the expected value of 12+log(O/H) is
$\sim$8.37 $\pm$ 0.15 dex. For disc galaxies with the visible metallicity
gradients, this parameter is usually adopted at the radial distance of reff/2.
Thus, taking into account rather small metallicity gradients in the
subluminous galaxies like NGC6503 ($\sim$0.02–0.03 dex r${}_{\rm eff}^{-1}$),
we expect the gas metallicity in the outer disc of NGC6503 at the level of
12+log(O/H) $\gtrsim$ 8.1–8.2 dex.

Therefore, if we accept the hypothesis of gas inflow from NGC6503 to the
extremely gas-poor predecessor of dTr KK242, with the subsequent triggered
episode of star formation, we should expect the metallicity of gas in the N
knot, corresponding to 12+log(O/H) $\gtrsim$8.1$\pm$0.15 dex. In Fig. 6, the
12+log(O/H) = 8.1 dex for Cloudy models with log(U) = –4.0 (red diamonds)
corresponds to the value of S2 $\sim$1.0 (log(S2) = 0), that is more than
2$\sigma$ larger than the nominal value S2 = 0.31.

These estimates adopt the gas metallicity of NGC6503 based on the relation
established by Berg et al. (2012) on the subsample of the late-type galaxies
within the Local Volume, and hence, should be nicely applicable to NGC6503.
However, as discussed in our papers (Pustilnik et al., 2016, 2020, 2021), this
sample is mostly related to the typical groups and their environs. For
galaxies with the same MB residing in the nearby voids, the gas metallicity
(or log(O/H)) is in average lower by 0.14 dex. For ’luminous’ galaxies similar
to NGC6503, this effect is smaller, $\sim$0.1 dex (Pustilnik et al., 2021,
Fig.1). Since NGC6503, as described in Section 4.2, resides in the void
Dra–Cep, we expect, that the above estimates of 12+log(O/H) should be reduced.
That is the expected value of gas O/H in the N knot, in the framework of the
hypothesis with the gas inflow from the outer disc of NGC6503, is 12+log(O/H)
$\gtrsim$ 8.0 dex.

We summarize this attempt to relate the highest possible value of S2 (about
two S2 nominal) and the respective value of 12+log(O/H) $\sim$7.8 dex for the
KK242 N knot with the ’metal-rich’ gas from the outer parts of NGC6503
(12+log(O/H) $\gtrsim$8.0 dex), as follows. The gap between the two extreme
possible values for the gas metallicity in KK242 and NGC6503 remains too
large. So that it is hard to reconcile this hypothesis with the available
data.

###  4.4 Fading stages of faint SF episodes and the problem of gas metallicity
estimate

In the light of the discussed above dwarf galaxies with only the weak tracers
of the recent SF episode, we would like to emphasize that this type of objects
should be numerous and widely spread among the low mass dwarfs. Due to their
low masses, their gas metallicity is expected to be in the low-Z regime.

Indeed, in a sizeable fraction of low mass late-type LSB galaxies, the
observed SFR is subtle. See, e.g., Kaisin & Karachentsev (2019) for dwarfs in
the Local Volume and Hauberg et al. (2015) for small gas-rich dwarfs selected
from the ALFALFA survey (Haynes et al., 2018) and IZw18C (Izotov et al.,
2001). The observed H$\alpha$ luminosities per individual Hii region indicate
a small number of the hot massive stars in such dwarfs. The LV dwarfs with the
lowest observed luminosities of L(H$\alpha$) of the order of $\lesssim$1036
erg s-1 correspond to the ionising photon fluxes Q0 of stars O9V and later.
Several XMP galaxies from our recent papers (Pustilnik et al., 2020, 2021)
fall to this regime as well. In addition, the new current and upcoming deep
sky surveys will drastically increase the number of such galaxies.

For an instantaneous SF episode, the probability to catch it in the early
phases (say, younger than $\lesssim$ 8–10 Myr) when sufficiently hot stars
(conditionally, O5V-O8V) are still alive666if they were really formed in the
modest total mass involved. and provide a high enough log(U) and Teff, is
lower than to catch a region with the exciting stars of O9V-B0V, with the
lower fluxes of the ionising photons and Teff. In such cases, if we wish to
obtain the estimates of gas metallicity, we need some alternative means.

For that low excitation conditions and the related very low fluxes of [Oiii]
lines, the possibility to use the ’strong-line’ empirical methods to estimate
their gas metallicity is hampered. Due to various observational selection
effects, such objects remain largely underexplored, in particular, in the
context of their gas metallicity.

We suggest to use for such low excitation Hii regions with the Hydrogen Balmer
series lines and only a few heavy element lines detected – [Oii]$\lambda$3727,
[Sii]$\lambda$6716,6731 doublets and probably [Nii]$\lambda$6584, the grid of
Cloudy package models with the low log(U) and low Teff as typical for Hii
regions ionized by the early B-type and late O-type stars.

The examples of such analysis for KK242 and DDO216 presented above, show that
this is feasible. The main pre-requisite of the successful application of such
models is a sufficiently good S-to-N ratio for the used heavy element line
fluxes.

The used here the Cloudy grids are based on the models of the central star
with Teff. The more advanced grids, with the modern stellar atmosphere models
and the stellar metallicity included, can give us an advanced mean to treat
spectra and derive a more reliable estimates of the gas metallicity in the low
excitation Hii regions in a large number of dwarf galaxies within the Local
Volume and its environs. This, in turn, should allow one to address the issue
of chemical evolution on a wider range of galaxy parameters.

The output of such model grids in the form of the relative line fluxes can be
used similar to the ’Counterpart’ method of Pilyugin et al. (2012) which seeks
in the reference database of the observed Hii regions with known direct O/H
for a combination of the relative line fluxes which is the closest to that in
the studied Hii region without detected [Oiii]$\lambda$4363 line.

###  4.5 Physical parameters and star formation

The dTr galaxy KK242 is very interesting in the context of its current and
recent star formation. Its gas mass is several times smaller than the typical
of the comparable stellar mass and luminosity late-type LSB dwarfs. Therefore,
if its Hi gas distribution is not strongly clumped, this suggests the reduced
column density. Indeed, as the VLA map in the Hi-line reveals (Karachentsev et
al., 2022), the peak Hi column density in this galaxy reaches only
3$\times$1019 atom cm-2. This is to compare with the typical threshold gas
column density for the onset of star formation in dwarf irregulars, blue
compact and LSB galaxies defined at the level of 1.0$\times$1021 atom cm-2 for
the linear scales of $\sim$0.5 kpc (Skillman, 1987; Taylor et al., 1994; Begum
et al., 2008; Ekta et al., 2008). This can be partly explained by the large
effective beam-size of $\sim 60\times 40$ arcsec2 (or 1.85 $\times$ 1.24 kpc),
which smears the higher density features at smaller linear scales. However,
the significant column gas underdensity of the KK242 SF region attracts the
special attention to this point.

The issue of the threshold column density for the onset of SF is not settled,
however (see, e.g., discussion in Ekta et al., 2008). The model calculations
(e.g. Schaye, 2004) predict a range for this parameter which depends on gas
mass fraction, pressure, its metallicity, ionising flux radiation. Therefore,
it will take much effort to understand whether the formation of the studied
complex of several massive hot stars in KK242 took place in a low density gas
under the special conditions.

The important factor of the SF episode onset is the proximity of KK242 to its
host NGC6503. As many N-body simulations indicate (e.g. Di Matteo et al.,
2007), the peak of the tidally induced SF episode occurs in a few hundred Myr
after the first pericenter passage. We can roughly estimate the time since the
passage of KK242 near the host. Taking the relative tangential component of
velocity ($\delta V_{\rm tang}$) approximately equal to the relative radial
velocity $\delta V_{\rm rad}\sim$100 km s-1, and the mutual projected distance
of $\delta r\sim$ 31 kpc, we estimate the respective time t${}_{\rm
passage}\sim$ 300 Myr. Therefore, the presence in the discussed SF region of
RHeB and BHeB stars, with ages of $\sim$100 Myr, does not contradict to the
assumption that the long-lasting very localised SF episode was triggered by
the strong tidal interaction with the massive host. To get a deeper insight
into gas properties involved in the SF episode at such atypical conditions, it
seems, one needs to wait for the ngVLA, which will allow one to obtain Hi data
simultaneously with the high sensitivity and suitable angular resolution.

##  5 Summary and conclusions

In this study we primarily interested in the determination of the radial
velocity of the ionised gas in KK242 in order to get its value independent on
the previous Hi observation. Thanks to the proper observational conditions, we
spatially resolved two faint emission regions (the N and S knots) within the
studied SF complex. Their appearance is rather different that pushed us to
analyze them individually. Thanks to the publicly available F606W and F814W
HST images of KK242, as well as of the photometry of its individual stars, we
were able to disentangle the exciting stars of the observed nebular emission.
In the S knot we also identify a nebula, a likely SN remnant.

Due to rather low S-to-N of the available spectra of this SF complex and
comparatively low effective temperatures of the ionising massive stars, the
only heavy element lines detected so far is the [Sii]$\lambda\lambda$6716,6731
doublet. Its flux ratio to that of H$\beta$ (parameter S2) can be used as a
rough empirical indicator of the gas metallicity. However, the S2 in our data
has the large uncertainty that allows rather wide range of O/H. If we compare
the observed S2 in the N knot of KK242 with the data for the sample of Hii
regions with the directly derived O/H (that is with the medium or high
excitation), it corresponds to values of 12+log(O/H) between 7.6 and 8.0 dex.

However, the examination of our Cloudy package grids with the wide range of
lg(U) and Teff of ionising stars, reveals the elevated values of S2 for the
’extremely’ low values of lg(U) $\sim$ –4.0 and Teff $\sim$ 25–30 kK. These
elevated values of S2 are consistent with the observed one for 12+log(O/H) as
low as $\sim$7.45 dex. Meanwhile, the mentioned above ’extremely’ low lg(U)
and Teff are consistent with those expected for B0V stars directly observed in
this region at the HST images. The possibility of that ’low’ value of O/H is
important, since this is expected for KK242 luminosity and stellar mass from
the reference relations for the LV late-type galaxies in Berg et al. (2012).
The gas metallicity is a crucial parameter for the choice between possible
evolutionary scenarios. The currently available estimate of O/H is consistent
with the case of the typical secular chemical evolution of late-type dwarfs as
possible predecessors of the dTr KK242. This conclusion remains valid if we
take into account that the evolution of KK242 took place in a void, and
therefore one expects it to be reduced.

We also examine an alternative scenario involving the higher metallicity gas
inflow from the outer disc of NGC6503 to the ’gas-free’ dE predecessor of
KK242 after its pericenter passage. From the estimates of the possible
metallicity of the in-flowed gas, this variant seems to be improbable due to
the substantial gap between the upper limit of the gas metallicity in KK242
and the lower limit of that in the outer part of NGC6503.

Therefore, the most likely scenario of the origin of KK242 as a void dTr,
combines its secular evolution as a void low-mass dIrr and the ’recent’
rapprochement and interaction with the much more massive host NGC6503. The
pericenter passage of KK22 several hundred Myr ago resulted in the tidal
stripping of its gas and triggered the episode of SF (e.g., Di Cintio et al.,
2021). The traces of this ’recent’ star formation are seen in the GALEX UV
images as light of stars with the ages of less than a few hundred Myr (Koda et
al., 2015) as well as BHeB and RHeB stars in the studied here region with the
faint H$\alpha$ emission.

Summarising all available data and the above analysis and discussion, we
arrive at the following conclusions.

  1. 1.

KK242 is a transition type dwarf residing in a nearby void. We report its BTA
spectroscopy and the new value of its radial velocity, Vhel = –66$\pm$10 km
s-1, based on the observed H$\alpha$ line in two adjacent regions of the star-
forming complex identified by Koda et al. (2015). This value is consistent
with that of the recently found faint Hi emission from KK242 (Karachentsev et
al., 2022) and is lower than the radial velocity of its host spiral NGC6503 by
$\sim$100 km s-1.

  2. 2.

The appearance of these two regions in the BTA 2d spectra of KK242 looks very
different due to the bright light of the ’unrelated’ Red Supergiant at the
projected angular distance of $\sim$0.7 arcsec from the H$\alpha$ intensity
peak of the Northern H$\alpha$ knot. On the HST images of KK242, we identify
the sources responsible for the visible H$\alpha$ emission within the BTA long
slit. The Northern region is a superposition of three Hii regions exciting by
the late O-type and early B-type stars at the mutual projected distances of
$\sim$0.7–1.0 arcsec (20–30 pc). The H$\alpha$ emission of the Southern region
is a superposition of an Hii region excited by an early B-star and of a
probable supernova remnant at a projected distance of $\sim$0.7 arcsec.

  3. 3.

Besides the early-type blue stars, we identify within the boundaries of this
SF complex three additional luminous stars. They are tentatively classified as
one RHeB star (the mentioned above Red Supergiant) and two BHeB stars. Their
colours and absolute magnitudes imply their ages of $\lesssim$100 Myr. This
suggests that the recent SF episode as traced by several massive young main-
sequence stars (O9-B1) in fact lasts in this location at least of $\sim$0.1
Gyr or so.

  4. 4.

Due to rather low S-to-N ratio spectrum of the Northern Hii region and its low
excitation, the only detectable metal lines appear those of the [Sii] doublet.
We use the parameter S2 (the flux ratio of [Sii] to that of H$\beta$) to
constrain the gas metallicity in this region. The direct comparison of the
nominal value of S2(KK242,N) = 0.31 with the Hii regions from the compilation
of Pilyugin et al. (2012), allows the wide range of 12+log(O/H) = 7.6–8.0 dex.
This range, however, is poorly consistent with the low 12+log(O/H)
$\sim$7.4$\pm$0.1 dex, expected for KK242 low stellar mass and luminosity in
the case it is treated as a dIrr which lost most of its gas.

  5. 5.

We undertake a deeper analysis of this situation taking into account the HST
data on the ionising stars’ parameters (O9V–B0V). We use the Cloudy package to
construct grids of the common line flux ratios in Hii regions with the wide
range of gas metallicities, ionisation parameter U and the effective
temperature of the central star. The observed parameter S2 in the Northern
knot of KK242 can be well consistent with the predicted in models with
12+log(O/H) $\sim$7.4 dex, if both log(U) and Teff are very low, $\sim$-4.0
and $\sim$25–30 kK, respectively. These log(U) and Teff are close to those
expected for the massive hot stars visible in this region.

  6. 6.

For the Southern H$\alpha$ knot, the higher S-to-N value of flux of the [Sii]
doublet indicates its elevated ratio relative to the flux of H$\alpha$. We
argue that since the main contribution to the emission of this knot comes from
the nearby nebula, a likely SN remnant, this elevated [Sii] emission is
related to the shock excitation in the SNR shell.

  7. 7.

We pay attention to the generalisation of the problem of gas metallicity
determination in common low-excitation low-metallicity Hii regions of LSB
dwarfs. In such regions, only a few heavy element emission lines are typically
observed and the use of the popular strong-line empirical estimators can be
impossible. We suggest to develop a grid of Cloudy package models representing
the observed line ratios for the wide range of gas and young star metallicity
when only a population with the ages of more than 10–15 Myr excites their
related Hii regions (late O and early B-type). This will give one a new
advanced mean to address the gas low metallicity in the dwarfs of the Local
Universe with the low/subtle current SF.

## Acknowledgements

The work was supported by the Russian Scientific Fund (RScF) grant No.
22-22-00654. The authors thank I.D. Karachentsev for sharing some results on
KK242 before publication. We also thank D.I. Makarov for the help with the use
of the publicly available HST data. We acknowledge the constructive
suggestions of the anonymous referee which helped us to improve and clear up
the paper contents. The authors acknowledge the allocation of the SAO DDT time
at BTA in November 2021. Observations with the SAO RAS telescopes are
supported by the Ministry of Science and Higher Education of the Russian
Federation. The renovation of telescope equipment is currently provided within
the national project ”Science and Universities”. This research is partly based
on observations made with the NASA/ESA Hubble Space Telescope obtained from
the Space Telescope Science Institute, which is operated by the Association of
Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.
These observations are associated with program SNAP-15922. We acknowledge the
use of the Cloudy photoionisation code to model the intensities of the common
emission lines in Hiiregions of the low excitation. We acknowledge the use for
this work of the database HyperLEDA777http://leda.univ-lyon1.fr. This research
has made use of the NASA/IPAC Extragalactic Database (NED) which is operated
by the Jet Propulsion Laboratory, California Institute of Technology, under
contract with the National Aeronautics and Space Administration.

## Data availability

The data underlying this article are available in the The Extragalactic
Distance Database (EDD)888http://edd.ifa.hawaii. edu/. The HST/ACS data used
in this article are available in the STScI data archive.

## References

  * Afanasiev & Moiseev (2005) Afanasiev V.L., Moiseev A.V., 2005, Astron. Lett., 31, 193
  * Afanasiev & Moiseev (2011) Afanasiev V.L., Moiseev A.V., 2011, BaltA, 20, 363
  * Allen et al. (2008) Allen M.G., Groves B.A., Dopita M.A., Sutherland R.S., Kewley L.J., 2008, ApJS, 178, 20
  * Anand et al. (2021) Anand G.S., Rizzi L., Tully R.B. et al. 2021, AJ, 162, 80
  * Aragon-Calvo & Szalay (2013) Aragon-Calvo M.A., & Szalay A.S., 2013, MNRAS, 428, 3409
  * Asplund et al. (2009) Asplund M., Grevesse N., Sauval A.J. Scott P., 2009, Ann.Rev.Astron.Astrophys., 47, 481
  * Begum et al. (2008) Begum A., Chengalur J.N., Karachentsev I.D., Sharina M.E., Kaisin S.S., 2008, MNRAS, 386, 1667
  * Blanc et al. (2013) Blanc G.A., Weinzirl T., Song M. et al., 2013, AJ, 145, 138
  * Berg et al. (2012) Berg D.A., Skillman E., Marble A. et al. 2012, ApJ, 754, 98
  * Di Cintio et al. (2021) Di Cintio A., Mostoghiu R., Knebe A., Navarro J.F., 2021, MNRAS, 506, 531
  * Di Matteo et al. (2007) Di Matteo P., Combes F., Melchior A.-L., Semelin B., 2007, A&A, 468, 61
  * Ekta et al. (2008) Ekta, Chengalur J.N., Pustilnik S.A., 2008, MNRAS, 391, 881
  * Ferland et al. (2017) Ferland G.J., Chatzikos M., Guzmán F. et al. (2017) Revista Mexicana de Astronomia y Astrofisica, 53, 385-438
  * Filippenko (1982) Filippenko A.V., 1982, PASP, 94, 715
  * Hauberg et al. (2015) Hauberg N.C., Salzer J.J., Cannon J.M., Marshall M.V., 2015, ApJ, 800, 121
  * Haynes et al. (2018) Haynes M.P. et al., 2018, ApJ, 861, 49
  * Hirschauer et al. (2016) Hirschauer A.S. et al., 2016, ApJ, 822, 108
  * Hsyu et al. (2017) Hsyu T., Cooke R.J., Prochaska J.X., Bolte M., 2017, ApJ Lett., 845, L22
  * Huchtmeier et al. (2000) Huchtmeier W., Karachentsev I.D., Karachentseva V.E., Ehle M., 2000, A&A Suppl., 141, 469
  * Izotov & Thuan (1999) Izotov Y.I., Thuan T.X., 1999, ApJ, 639
  * Izotov et al. (2001) Izotov Y.I., Chaffee F.H., Foltz C.B., Thuan T.X., Green R.F., Papaderos P., Fricke K.J., Guseva N.G., 2001, ApJ, 560, 222
  * Izotov, Thuan & Guseva (2012) Izotov Y.I., Thuan T.X., Guseva N.G., 2012, A&A, 546, A122
  * Izotov et al. (2018) Izotov Y.I., Thuan T.X., Guseva N.G., Liss S.E., 2018, MNRAS, 473, 1956
  * Izotov, Thuan & Guseva (2019) Izotov Y.I., Thuan T.X., Guseva N.G., 2019, MNRAS, 483, 5491
  * Jacobs et al. (2009) Jacobs B.A., Rizzi L., Tully R.B., Shaya E.J., Makarov D.I., Makarova L.N., 2009, 138, 332
  * Kaisin & Karachentsev (2019) Kaisin S.S., Karachentsev I.D., 2019, Astropys.Bull., 74, 1
  * Karachentseva & Karachentsev (1998) Karachentseva V.E., Karachentsev I.D., 1998, A&A Suppl., 127, 409
  * Karachentsev et al. (2013) Karachentsev I.D., Makarov D.I., Kaisina E.I., 2013, AJ, 145, 101
  * Karachentsev et al. (2014) Karachentsev I.D., Makarova L.N., Tully R.B., Wu P.-F., Kniazev A.Y., 2014, MNRAS, 443, 1281
  * Karachentsev et al. (2022) Karachentsev I.D., Cannon J.M., Fuson J., Inoue J.L., Tully R.B., Anand G.S., Kaisin S.S., 2022, AJ, 163, id.51
  * Koda et al. (2015) Koda J., Yagi M., Komiyama Y. et al., 2015, ApJ Letters, 802, L24
  * Kopsacheili, Zezas & Leonidaki (2020) Kopsacheili M., Zezas A., Leonidaki I., 2020, MNRAS, 491, 889
  * Mateo (1988) Mateo M.L., 1998, ARAA, 36, 435
  * McQuinn et al. (2011) McQuinn K.B.W., Skillman E.D., Dalcanton J.J., Dolphin A.E., Holtzman J., Weisz D.R., Williams B.F., 2011, ApJ, 740, id.48
  * McQuinn et al. (2012) McQuinn K.B.W., Skillman E.D., Dalcanton J.J., Cannon J.M., Dolphin A.E., Holtzman J., Weisz D.R., Williams B.F., 2012, ApJ, 759, id.77
  * Osterbrock & Ferland (2006) Osterbrock D.E., Ferland G.J., 2006, Astrophysics of Gaseous Nebulae and Active Galactic Nuclei, 2nd edition, University Science Books, Sausalito, California
  * Pilyugin et al. (2012) Pilyugin L.S., Grebel E.K., Mattsson L., 2012, MNRAS, 424, 2316
  * Pustilnik, Martin (2016) Pustilnik S.A., Martin J.-M., 2016, A&A, 596, A86
  * Pustilnik, Tepliakova (2011) Pustilnik S.A., Tepliakova A.L., 2011, MNRAS, 415, 1188
  * Pustilnik et al. (2016) Pustilnik S.A., Perepelitsyna Y.A., Kniazev A.Y., 2016, MNRAS, 463, 670
  * Pustilnik et al. (2019) Pustilnik S.A., Tepliakova A.L., Makarov D.I., 2019, MNRAS, 482, 4329
  * Pustilnik et al. (2020) Pustilnik S.A., Kniazev A.Y., Perepelitsyna Y.A., Egorova E.S., 2020, MNRAS, 493, 830
  * Pustilnik et al. (2021) Pustilnik S.A., Egorova E.S., Kniazev A.Y., Perepelitsyna Y.A., Tepliakova A.L., Burenkov A.N., Oparin D.V., 2021, MNRAS, 507, 944
  * Ramachandran et al. (2019) Ramachandran V., Hamann W.-R., Oskinova L.M. et al., 2019, A&A, 625, A104
  * Schaye (2004) Schaye J., 2004, ApJ, 609, 667
  * Schlafly & Finkbeiner (2011) Schlafly E.F., Finkbeiner D.P., 2011, ApJ, 737, 103
  * Skillman (1987) Skillman E.D., 1987, in NASA Conf. Publ Vol. 2466, Star Formation in Galaxies, Washington, p. 263
  * Skillman et al. (1997) Skillman E.D., Bomans D.J., Kobulnicky H.A., 1997, ApJ, 474, 205
  * Skillman et al. (2003) Skillman E.D., Côté S., & Miller B.W., 2003, AJ, 125, 593
  * Skillman et al. (2013) Skillman E.D., Salzer J.J., Berg D.A. et al., 2013, AJ, 146, 3
  * Taylor et al. (1994) Taylor C.L., Brinks E., Pogge R.W., Skillman E.D., 1994, AJ, 107, 971
  * Weisz et al. (2011) Weisz D.R., Dalcanton J.J., Williams B.F., et al. 2011, ApJ, 739, 5

##  Appendix A [Sii]$\lambda\lambda$6716,6731 doublet: the range of its
strength via Cloudy

In this Appendix we use the Cloudy.17 version of the Cloudy radiative transfer
code, as described by Ferland et al. (2017), to perform calculations of the
possible range for the value of parameter S2 (flux ratio of the doublet
[Sii]$\lambda\lambda$6716,6731 to that of H$\beta$) for a range of Hii region
gas metallicities. We took Oxygen abundances of 12+log(O/H) = 7.00, 7.17,
7.35, 7.53, 7.70, 7.90 and 8.04 dex.

The relative abundances C/O, N/O, Ne/O, Si/O, S/O, Ar/O, Fe/O are adopted as
the averages found in the systematic study of 54 supergiant Hii regions with
the range of 12+log(O/H) = 7.1 – 8.2 dex in the paper by Izotov & Thuan
(1999). Since the abundance of O/H is set in the units of the Solar one, we
adopt 12+log(O/H)⊙= 8.69 dex according to Asplund et al. (2009).

We construct a grid of models with the range of the ionisation parameter U
(log(U) from -4.0 to -1.0, with the step of 0.5, and the effective temperature
of the central star (black-body) Teff from 25 to 50 kK, with the step of 5 kK.
We then apply these results to the observational data for the Northern knot in
KK242 (Fig. 6 and Section 4.3.3) and attempt to constrain its metallicity from
the observed parameter S2.

We run Cloudy in the mode ”sphere” with the adopted input parameters of the
inner radius of R0 = 1017 cm and the number density of the hydrogen nuclei of
n(H)=10 cm-3. Cloudy then computes the structure of the photoionised region by
requiring that the density be constant. The outer radius is assumed to be the
one where the gas temperature falls to 4 kK since the colder gas practically
does not produce optical emission lines. The resulting geometry of all models
was closed, that is, the gas covers most of the central ionising source. The
iterations were carried out until the optical depth of the line and continuum
became stable. For the gas element abundances, we do not take into account a
presumably small fraction of material which is possibly locked into grains. We
remark for clarity, that the ionisation parameter U shown on the X-axes of
Figs. A1 and A2, is obtained as a result of a model and relates to the
Strömgren radius, in difference with an input parameter of a model at the
inner radius. That is, it directly corresponds to the parameter U, estimated
for a B0V star in Section 4.3.4.

Figure 7:  Plots showing variation of parameter S2 in an Hii region as a
function of the ionisation parameter log(U) for ionising central stars with
the range of Teff from 25 to 50 kK for three values of gas O/H, 12+log(O/H) =
7.35, 7.53 and 7.70, from top to bottom, respectively, as obtained with the
package Cloudy. For the fixed O/H, the S2 parameter appears a factor of 2 or
so larger at log(U) $\sim$ –4.0 than for Hii regions with log(U) $\gtrsim$
–2.5, typical of the observed for O/H (dir) (see Fig. 8).

Figure 8:  Similar plots as in Fig. 7 showing variation of the line flux ratio
[Oiii]$\lambda$4363 to H$\beta$. For T${}_{\rm eff}\lesssim$ 35 kK, this ratio
exceeds the conditional level of 0.01 (for Hii regions with the direct O/H
determination) only for models with the ionisation parameter log(U)
$\gtrsim-3.0$.