# Physics and Chemistry of Radiation Driven Cloud Evolution. [C II] Kinematics
of IC 59 and IC 63

Miranda Caputo Ritter Astrophysical Research Center, University of Toledo
Dept. of Physics and Astronomy, 2801 W. Bancroft St. Toledo, Ohio 43606, USA
SOFIA Science Center, USRA, NASA Ames Research Center, M.S.-12, N232, Moffett
Field, CA 94035, USA  Archana Soam Indian Institute of Astrophysics, II Block,
Koramangala, Bengaluru 560034, India  SOFIA Science Center, USRA, NASA Ames
Research Center, M.S.-12, N232, Moffett Field, CA 94035, USA  B-G Andersson
SOFIA Science Center, USRA, NASA Ames Research Center, M.S.-12, N232, Moffett
Field, CA 94035, USA  Remy Dennis  Santa Clara University, 500 El Camino Real,
Santa Clara, CA 95053, USA  Ed Chambers SOFIA Science Center, USRA, NASA Ames
Research Center, M.S.-12, N232, Moffett Field, CA 94035, USA  Rolf Güsten Max
Planck Institut für Radio Astronomie, Bonn, Germany  Lewis B.G. Knee Herzberg
Astronomy and Astrophysics Research Centre, National Research Council of
Canada, 5071 West Saanich Road, Victoria, BC, V9E 2E7, Canada  Jürgen Stutzki
I. Physikalisches Institut der Universität zu Köln, Zülpicher Straße 77,
D50937 Köln, Germany

###### Abstract

We used high-resolution [C II] 158 $\micron$ mapping of two nebulae IC 59 and
IC 63 from SOFIA/upGREAT in conjunction with ancillary data on the gas, dust,
and polarization to probe the kinematics, structure, and magnetic properties
of their photo-dissociation regions (PDRs). The nebulae are part of the Sh
2-185 H II region illuminated by the B0 IVe star $\gamma$ Cas. The velocity
structure of each PDR changes with distance from $\gamma$ Cas, consistent with
driving by the radiation. Based on previous FUV flux measurements of, and the
known distance to $\gamma$ Cas along with the predictions of 3D distances to
the clouds, we estimated the FUV radiation field strength (G0) at the clouds.
Assuming negligible extinction between the star and clouds, we find their 3D
distances from $\gamma$ Cas. For IC 63, our results are consistent with
earlier estimates of distance from Andersson et al. (2013), locating the cloud
at $\sim$2 pc from $\gamma$ Cas, at an angle of 58∘ to the plane of the sky,
behind the star. For IC 59, we derive a distance of 4.5 pc at an angle of 70∘
in front of the star. We do not detect any significant correlation between the
orientation of the magnetic field (Soam et al., 2017) and the velocity
gradients of [C II] gas, indicating a moderate magnetic field strength. The
kinetic energy in IC 63 is estimated to be order of ten higher than the
magnetic energies. This suggests that kinetic pressure in this nebula is
dominant.

ISM: Clouds – Submillimeter: ISM – ISM: lines and bands – ISM: individual (IC
63) – ISM: individual (IC 59) – (ISM:) photon-dominated region (PDR)

††journal: ApJ

##  1 Introduction

Figure 1: RGB image of the Sh 2-185 H II region made from WISE W1
(3.4$\micron$) in blue, W3 (12$\micron$) in green, and W4 (22$\micron$) in
red.

Radiation from newly formed, hot, high-mass O and B stars can give rise to a
number of dynamical and chemical effects in surrounding material. Ionizing
radiation from these young stars can cause exposed material in the surrounding
cloud to photoevaporate and ionize the inner boundaries of expanding HII
regions. Existing density enhancements on the peripheries of expanding H II
regions create bright-rimmed clouds (BRCs). The bright rim produced by the
recombination radiation from the ionization front (IF) on their surface facing
the ionizing source.

High energy radiation from ionising sources causes feedback processes in the
surrounding medium. This feedback can be positive or negative; causing
triggered star formation or quenching of star formation respectively. A
systematic investigation is needed on injection of mechanical energy and
radiative heating efficiency in regions dominated by the different processes
(stellar winds, thermal expansion, radiation pressure). However, some
numerical simulations, such as Lancaster et al. (2021), shed controversy on
the importance of stellar winds, leaving mainly thermal expansion and
photoevaporation. The [C II] line uniquely probes the kinematics of the gas
exposed to the mechanical energy input by massive stars and therefore directly
measures the mechanical energy injection into the medium. The observations of
the [C II] line suggest that that winds could dominate for early type stars
while thermal expansion dominates for later type stars, e.g. Pabst et al.
(2020). The dominant processes in PDRs depends not only on the stellar
spectral type, but also the evolutionary state of the system, internal
pressures, and the geometry and distance of the clouds from the star. The case
of IC 59 and IC 63 presents an interesting example in which the feedback from
a B0 star is dominated by photevaporation. The radiative feedback processes
acts directly on the PDR while mechanical feedback is transmitted through the
PDR to the cloud interiors.

Associated with these H II regions and BRCs, photo-dissociation regions, or
photon-dominated regions (PDRs), can be described as the regions in the
interstellar medium (ISM) where UV radiation dominates the photochemistry
(Hollenbach & Tielens, 1997; Wolfire et al., 2022). These PDRs are found in
the transitional regions between the ionized, atomic material and the colder,
denser, molecular material in clouds. UV radiation can infiltrate the denser
gas in these PDR regions exciting, ionizing, and affecting the gas motion,
inducing chemical, thermal, and dynamical changes, and, possibly, triggering
star formation. These features make PDRs important laboratories for probing
the physics, chemistry, and evolution of the ISM.

Amongst the nearest of well-defined systems of PDRs is the Sh 2-185 H II
region created by the B0 IVe star $\gamma$ Cas (Figure 1), located at a
distance of $190\pm 20$pc, consisting of the IC 63 and IC 59 nebulae
(Osterbrock, 1957; France et al., 2005; Miao et al., 2010; Andersson et al.,
2013; Soam et al., 2017). Because of its brightness and proximity to the sun,
Sh 2-185 is an ideal laboratory to better test the modern theoretical PDR
models and to understand their structure and kinematics.

The brighter of these clouds, IC 63, has been observed through various studies
and surveys and is composed of several clumps with varying characteristics.
Jansen et al. (1994, 1995, 1996a, 1996b) performed an extensive, multi-line
tracer survey of IC 63 and interpreted their relatively low spatial resolution
data with comprehensive physical and chemical modeling without resolving the
structure of the cloud itself. Using interferometric maps in $\rm HCO^{+}$
(J=1-0), Polehampton et al. (2005) showed a highly compressed ridge on the
star facing side of IC 63 which is consistent with numerical models of BRCs
(Miao et al., 2009). Fleming et al. (2010) used $\rm H_{2}$ (0-0) S(2) through
S(5) excitation to show a temperature gradient across the cloud. Due to the
shape of IC 59 not being represented by most BRC models (Miao et al., 2009),
Miao et al. (2010) proposed and modeled an idealized M-type BRC morphology
based on IC 59 structure and supported the conclusion from Karr et al. (2005)
that there is no active star formation in IC 59.

Andrews et al. (2018) used infrared emission from both IC 63 and IC 59 to
study the PAHs in these PDRs. They concluded that both IC 63 and IC 59 are
experiencing photo-evaporation due to radiation from $\gamma$ Cas and that the
tips of each nebula are further away from $\gamma$ Cas than their respective
projected distances. They also used archival Herschel and Spitzer maps and
line spectroscopy of IC 63 and IC 59, using spectral energy distributions to
measure the FUV radiation field strength. They performed PDR modeling of the
[C II] and [O I] lines and the infrared continuum to derive the density and
temperatures of these clouds.

Along with surveys of gas and dust tracers, another of the many ways in which
these clouds have been studied is through starlight polarization observations
to investigate magnetic fields. PDRs are an ideal environment for the study of
grain alignment mechanisms, in particular the radiative alignment torque (RAT)
theory (Lazarian & Hoang, 2007; Andersson et al., 2013; Hoang et al., 2015).
Soam et al. (2017) mapped the magnetic fields in a region containing IC 63 and
IC 59 using optical polarization measurements. The magnetic fields were found
to be ordered. In IC 63, the field lines were found almost parallel to the
direction of illuminating radiation and almost perpendicular in case of IC 59.
These investigations were done to understand the magnetized evolution of these
nebulae. Polarization efficiency and collisional disalignment of dust grains
in IC 63 are studied by Soam et al. (2021d) and Soam et al. (2021a),
respectively. The gradients in temperature and densities in the PDR region of
IC 63 are also studied by Soam et al. (2021c) using pure rotational molecular
hydrogen observations.

High sensitivity observations of a PDR line-tracer at high spectral resolution
is necessary in order to probe the kinematics of the region. SOFIA upGREAT
velocity maps of [C II] are ideal for tracing PDRs. In this paper, our focus
is to understand the kinematics, structure, and geometry of these nebulae
using [C II] emission.

The paper is structured as follows: Section 2 describes the observations and
data reduction. Section 3 presents the detailed analysis. The results of the
gas and dust emissions and cloud kinematics, magnetic field and velocity
gradients, and the FUV radiation strength estimation in IC 63 are given in
Section 4. In Section 5, we discuss our findings and Section 6 summarizes our
results and describes proposed further work.

##  2 Data acquisition and reduction

We observed [C II] 158 $\micron$ line emission toward IC 63 and IC 59 using
the upgraded German Receiver for Astronomy at Terahertz Frequencies (upGREAT)
(Heyminck et al., 2012; Risacher et al., 2016) on board the Stratospheric
Observatory for Infrared Astronomy (SOFIA) (Young et al., 2012). The upGREAT
Low Frequency Array (LFA) is a 14-pixel array (two 7-pixel arrays with
orthogonal polarizations) that observes at $\sim$ 2 THz. Observations of both
sources were made as part of SOFIA project 05_0052 (PI: B-G Andersson).

The IC 63 observations were made on a $\sim$50-minute flight leg on 1 February
2017. To create the map, we used the total power on-the-fly mode (TP OTF),
scanning in the N-S direction. The map size was $\rm 288^{\prime\prime}$ x
$\rm 252^{\prime\prime}$, as seen by the central LFA pixel. The step size
between spectral readouts along each scan row was $\rm 6^{\prime\prime}$, and
the spacing between scan rows was $\rm 6^{\prime\prime}$. The integration time
in each readout was 0.7 s pixel-1, which translates to $\sim$10 seconds of on-
source integration time, given the spatial multiplexing advantage of 14
pixels.

The IC 59 observations were carried out during six flight legs in 2017, on 8,
9, 10, 14 February and 13 and 15 June. The total flight leg time for these six
legs was $\sim$3.5 hours. The IC 59 region was covered with two different TP
OTF maps. The first, larger map is $\rm 600^{\prime\prime}$ x $\rm
540^{\prime\prime}$ as seen by the central LFA pixel, and covers the entire
nebula. The second, smaller map is $\rm 468^{\prime\prime}$x$\rm
294^{\prime\prime}$, and is rotated by 25∘ (counter-clockwise) relative to the
J2000 coordinate frame. The sky position of the second map was optimized to
cover the [C II] emission detected in the first map. Both IC 59 maps have $\rm
6^{\prime\prime}$ between readouts along each scan row, and $\rm
6^{\prime\prime}$ separation between rows. The total on-source integration
time per map point in the combined map is $\sim$14 seconds.

All of the data was calibrated by the GREAT instrument team using their
kalibrate software (Guan et al., 2012). Main beam efficiencies for each pixel
were applied (values ranged from 0.59 to 0.68). We fit second order baselines
to a narrow spectral region (-21 to +21 km s-1 for both IC 63 and IC 59) after
masking out the line region (-4 to +4 km s-1 for IC 63, -5 to +5 km s-1 for IC
59). The final data cubes were gridded onto $\rm 7.5^{\prime\prime}$ pixels,
and spectrally smoothed to 0.4 km s-1. To remove spectral baselines and create
the final data cube, we used the CLASS111CLASS is part of the Grenoble Image
and Line Data Analysis Software (GILDAS; Pety, 2005), which is provided and
actively developed by IRAM, and is available at
http://www.iram.fr/IRAMFR/GILDAS software package.

We also used $\rm HCO^{+}$ (J=1-0) position-position-velocity maps observed
with CARMA222The Combined Array for Research in Millimeter-wave Astronomy.
https://en.wikipedia.org/wiki/Combined_Array_for_Research_in_Millimeter-
wave_Astronomy (Soam et al., 2021b), and molecular hydrogen $\rm H_{2}$ (1-0)
S(1) mapping from CFHT333Canada France Hawaii Telescope.
https://www.cfht.hawaii.edu/ (Andersson et al., 2013) in conjunction with low-
resolution $\rm{}^{12}CO$ (1-0) data from TRAO444Teaduk Radio Astronomical
Observatory at the Korea Astronomy and Space Science Institute.
https://trao.kasi.re.kr/main.php as part of a molecular line survey of 16 BRCs
by Soam et al. (in prep.) to help in studying the gas locations at different
opacities. Visual extinction data was obtained from a Vilnius-system
555VILNIUS is a medium-band, seven-color photometeric system(UPXYZVS).
Archival 70 and 250 -$\micron$ maps from Herschel PACS and SPIRE, WISE 22
$\micron$ maps, Spitzer IRAC 8 $\micron$, and IPHAS666The International
Photometric H$\alpha$ Survey of the northern galactic plane.
http://www.iphas.org/ H$\alpha$ (Drew et al., 2005; Barentsen et al., 2014)
maps of the regions were used to better understand the dust properties of the
clouds.

##  3 Analysis

###  3.1 Dust and Gas Emissions

Figure 2: Top panel: three-color, RGB image of IC 63 made with SOFIA upGREAT
[C II] integrated intensity map in red, CFHT $\rm H_{2}$ (1-0) S(1) in green,
and CARMA $\rm HCO^{+}$ (J=1-0) integrated intensity in blue. Middle panel:
RGB with [C II] in red and $\rm H_{2}$ (1-0) S(1) in green like above, but
blue is IPHAS H$\alpha$ emission. Lower panel: Model of IC 63 with naming
conventions.

Figure 3: Same as Figure 2 but for IC 59 and the blue in the upper panel is
$\rm{}^{12}CO$ (1-0) integrated intensity. Remnants of the imperfect star
subtraction and chip boundaries can be seen in the CFHT $\rm H_{2}$ (1-0)
S(1), green, emission.

Figure 2 shows two RGB images of IC 63 created with the upGREAT [C II]
integrated intensity map in red, CFHT $\rm H_{2}$ (1-0) S(1) map in green,
CARMA $\rm HCO^{+}$ (J=1-0) integrated intensity map in blue in the top panel,
and IPHAS H$\alpha$ emission in blue in the middle panel.

Figure 3 shows RGB images of IC 59 with [C II] in red, $\rm H_{2}$ (1-0) S(1)
in green, TRAO $\rm{}^{12}CO$ (1-0) integrated intensity map in blue in the
upper panel and as IPHAS H$\alpha$ in blue in the middle panel. The intensity
levels for each color in Figures 2 and 3 were chosen to provide a balance
between signal-to-noise and the desire to best represent all the data in the
images. The CFHT $\rm H_{2}$ (1-0) S(1) and the IPHAS H$\alpha$ images were
star subtracted.777Images were star subtracted thanks to the program
Starnet++, found at https://sourceforge.net/projects/starnet/.

The lower panels in Figures 2 and 3 illustrate the regions and naming
conventions used in the rest of this paper for IC 63 and IC 59, respectively.

Figure 4 shows integrated intensity maps of [C II] and $\rm HCO^{+}$ (J=1-0),
and dust emission maps from Herschel PACS 70 and SPIRE 250 $\micron$, WISE 22
$\micron$, and Spitzer IRAC 8 $\micron$ for IC 63. Panel (c) is the $\rm
HCO^{+}$ (J=1-0) integrated intensity map from panel (b) zoomed-in and with
the intensities stretched in order to show the fainter emission near the
second clump that is only visible when saturating the high emission region to
the west. Figure 5 shows the same maps as Figure 4 for IC 59 but without $\rm
HCO^{+}$ (J=1-0) maps.

Each map from Figures 4 and 5 has the corresponding beam size plotted in the
lower left corner and a physical scale for reference (0.01 pc for IC 63 and
0.1 pc for IC 59) on the lower right, assuming the clouds are 200 pc away. All
maps other than the [C II] integrated intensity map are over-plotted with
smoothed black or white contours of the corresponding [C II] integrated
intensity map with levels at [10, 15, 20, 25, 30, 35, 40] K km s-1,
$\sim$20-80% of the maximum integrated intensity of 50 K km s-1 for IC 63 and
[10, 13, 16, 20] K km s-1, $\sim$33-66% of the maximum integrated intensity of
30 K km s-1 in IC 59.

Figure 4: False color images of dust tracers and integrated intensity maps of
$\rm HCO^{+}$ (J=1-0) and [C II]. Dust tracer maps are Spitzer IRAC 8
$\micron$, WISE 22 $\micron$, and Herschel PACS 70 $\micron$ and SPIRE 250
$\micron$ maps. Overlaid black contours show smoothed [C II] integrated
intensity, panel (a), at levels [10, 15, 20, 25, 30, 35, 40] K km $\rm
s^{-1}$. Panel (c) is a zoomed in and stretched image of panel (b). All panels
are labeled on the top right corner with the related map name, lower left with
beam size, and lower right with a 0.01 pc scale reference except for panel
(c), where this is located in the upper left. Panel (a) also has a white arrow
showing the plane of the sky projection from $\gamma$ Cas as reference.

Figure 5: Same emission maps from Figure 4 but for IC 59 and without $\rm
HCO^{+}$ (J=1-0). Overlaid black and white contours show smoothed [C II]
integrated intensity, panel (a), at levels [10, 13, 16, 20] K km $\rm s^{-1}$.
All panels are labeled on the lower right with a 0.1 pc scale reference.

###  3.2 Velocity Channel Maps

Figure 6: False color channel map of [C II] for IC 63. The corresponding
velocity for all channels are marked in the upper right-hand corner of each
panel.

Figure 7: Same as Figure 6 but for IC 59

Figure 6 shows a velocity channel map of [C II] in IC 63 obtained from our
SOFIA/upGREAT position-position-velocity data, moving from the blue-shifted
velocity component of $\sim$-2 km $\rm s^{-1}$ on the top left to the blue-
shifted velocity component $\sim$3.4 km $\rm s^{-1}$ on the bottom right.
Contours are at levels 4 to 20 K km $\rm s^{-1}$ in steps of 4 K km $\rm
s^{-1}$. The velocity channel map of [C II] for IC 59 is shown in Figure 7
with the same general properties as Figure 6 but with velocities from -2.5 km
s-1 to $\sim$2.9 km s-1, labels on the lower right, and contours at levels 2,
5, and 8 K km $\rm s^{-1}$.

###  3.3 Position-Velocity Diagrams

Figure 8: Upper panel: gray-scale [C II] integrated intensity map of IC 63
with arrows indicating where and in which direction PV slices were taken.
Middle panel: PV diagrams for the six slices, shown in upper plot, are plotted
over the same velocity range, from -3 to 4 km s-1, and represented by the same
color scale. PV diagrams representing slices along the tails are on the top
row while those representing the clumps are on the bottom. Lower panel: PV
diagrams for slice 5 in $\rm HCO^{+}$ (J=1-0) and formatted like above. The
image on the left has had the colors stretched in order to see the the
velocity intensity peak around an offset of 2.5$\arcmin$. The image on the
right has a more even color map.

Figure 9: Same as Figure 8 but for IC 59 and with PV slices plotted from -4 to
3 km s-1.

The upper panels of Figures 8 and 9 show gray-scale [C II] integrated
intensity maps with numbered arrows where slices have been taken through the
position-position-velocity upGREAT [C II] data cubes of IC 63 and IC 59. The
position-velocity (PV) diagrams from these slices are shown in the lower
panels of the figures, with velocities from $\sim$-3 through 4 km $\rm s^{-1}$
for IC 63 and $\sim$-4 through 3 km $\rm s^{-1}$ for IC 59. $\rm HCO^{+}$
(J=1-0) data from slice 5 in IC 63 is also shown on the bottom panel for
Figure 8 from -1 to 1.5 km s-1 in two different color stretches to show the
slight emission at an offset of $\sim$2.5′. The offset begins at $\rm
0^{\prime}$ from the base of each arrow from the upper panels and concludes at
the tips of the arrow heads.

##  4 Results

###  4.1 Dust and Gas Relations

####  4.1.1 IC 63

As can be seen in Figures 2 and 4, the highest intensity regions of IC 63 are
the fishhook feature inside the main clump, the small clump, and the front
edge in those maps where the resolution is good enough to see it. The other
noticeable, but less intense, features are the upper and lower tails that
appear to stream from the different clumps. The 250 $\micron$ map from
Herschel SPIRE, panel (g) in Figure 4, has a background source to the west of
the PDR that is not seen in any other map shown in Figure 4. This object is
not being considered in our analysis.

In low-resolution (44′′) observations of IC 63 using CO (J=1-0) from TRAO, the
emission was seen in the main clump, but the small clump was not detected
(Soam et al., private communication). The small clump is detected in $\rm
HCO^{+}$ (J=1-0) and [C II] emissions .

The dense gasses, like $\rm HCO^{+}$ (J=1-0), mainly trace the fishhook
feature while the ionized gas, traced by H$\alpha$, appears around the areas
of the lowest optical depth, such as the western edges of the main and small
clumps as well as the upper tail. H$\alpha$ is also present over the majority
of the main clump at a slightly lower emission.

The tail features lie slightly off of the radial line from $\gamma$ Cas found
to the southwest (see white arrow in Figure 4 panel (a)). Andersson et al.
(2013) argued that, if the angle discrepancy between the projected radial
vector from $\gamma$ Cas and the angle of the tails is due to projection
effects, the line between the star, $\gamma$ Cas, and the nebula, IC 63, makes
an angle of $\sim$58∘ with respect to the plane of the sky.

####  4.1.2 IC 59

IC 59, seen in Figures 3 and 5, has multiple regions of high intensities. The
brightest region is the upper arm, followed by the main body and the head. The
eastern and western regions of the lower arm are the least bright regions. The
TRAO observations of $\rm{}^{12}CO$ (1-0) toward IC 59 only traces the upper
arm area, while the H$\alpha$ traces only the bottom ridge of the nebula
without overlapping with any of the other tracers, unlike what is seen in IC
63. A slight enhancement in the H2 fluorescence can be seen on the star-ward
side of the CO detection, reminiscent of the much stronger detection in IC 63.

###  4.2 Cloud Velocities

####  4.2.1 IC 63

One of the most noticeable features of Figure 6 is the intensity shift from
west to east as the velocities move from negative to positive. The most blue
shifted velocity region of the nebula is the western edge of the main clump,
including the fishhook and front edge, which also happens to be where
H$\alpha$ and $\rm H_{2}$ (1-0) S(1) are dominant (see Figure 2). This region
disappears in the more positive velocities, where the rest of the cloud is
visible.

As can also be seen with the PV diagrams in Figure 8, the small clump, slice
4, seems to span the largest velocity ranges from $\sim$-1 km s-1 through 3 km
s-1 while the main clump/fishhook, slice 6, seems to only range from $\sim$-2
km s-1 through 1 km s-1. The wide range in velocities covered by the small
clump leads to a larger integrated intensity, as seen in Figure 4 panel (a),
even though the peak intensities never surpass those of the fishhook. This
could mean that the small clump is less dense and cooler than the fishhook
region.

Slice 5 in Figure 8 shows that the main clump region, including the fishhook
and front edge, is much more blue shifted in velocity than the small clump,
with average velocities of $\sim$0 and 1.5 km s-1, respectively. The location
of the $\rm HCO^{+}$ (J=1-0) emission coincides with the emission of [C II].

Slice 2 shows that the tails are somewhat blue shifted in velocity along the
outer edges of the nebula, where some H$\alpha$ is seen in Figure 2, while
being redshifted towards the area in between the two tails.

####  4.2.2 IC 59

The velocity channel map of [C II] in IC 59, shown in Figure 7, seems to
follow similar patterns to IC 63 in Figure 6, but in opposite velocity
directions. The majority of the cloud is blue shifted in velocity and clearly
visible until -0.5 km s-1 where it begins to disappear. The main body, eastern
lower arm, and the upper arm are the only regions that persist into the
positive velocities. The highest intensity region, the upper arm, goes $\sim$1
km s-1 further into the positive velocities than any other region.

The upper arm of IC 59, where CO is detected and shown in blue in the upper
panel of Figure 3, is comparable to the main clump/fishhook region of IC 63
because these are the densest regions of each cloud. This is also where a
faint H2 fluorescent ridge can be seen. Comparing these two high density
regions in each cloud we find a contrast in their movements compared to the
rest of the clouds. The high density region in IC 63 contains, and is
surrounded by, the most negative [C II] velocities seen in the cloud while we
see the opposite in IC 59 with the most positive velocities seen in and around
the high density region of the upper arm.

Slice 1 of Figure 9 shows that the outside of the two arms are redshifted in
velocity, while the space between them is blue shifted. In comparison, slice 2
of Figure 8 shows that IC 63 is the opposite, where the outside of the tails
are blue shifted and the area in between is redshifted. This suggests that the
ionized gas towards the outside of both the arms and tails are traveling in
opposite directions from our perspective.

###  4.3 Spectral line features in different regions of the nebulae

Figure 10: Upper panel: boxed boundaries for the four main regions of IC 63
overlaid on the [C II] integrated intensity map, shown in gray-scale. Middle
panel: averaged line extraction spectra of [C II], black, from each of the
four regions from above with a Gaussian fitted in the same color as the region
box it represents. Lower panel: averaged line extraction spectra from $\rm
HCO^{+}$ (J=1-0) over the same regions and colors as [C II]. Fit parameters
for each Gaussian are shown in Table 1

.

Figure 11: Same as Figure 10 but for five regions in IC 59 and with extracted
spectra filled in gray. Fit parameters for each Gaussian are shown in Table 2.

The lower panels of Figures 10 and 11 show spectra that have been extracted
from the SOFIA/upGREAT [C II] position-position-velocity cube regions for IC
63 and IC 59, respectively, along with the spectra for $\rm HCO^{+}$ (J=1-0)
from these same regions in IC 63. The regions are over plotted and numbered on
the gray-scale [C II] integrated intensity maps in the upper panels of each
figure. Each of the spectra in the lower panels have been fitted with a single
Gaussian function to derive their quantitative properties, listed in Tables 1
and 2 for IC 63 and IC 59, respectively.

Table 1: Table of the parameters for the Gaussian fitted to the [C II] and $\rm HCO^{+}$ (J=1-0) velocity spectra of IC 63 for each region in Figure 10 Region | $\rm V_{peak}\pm\sigma_{V_{peak}}$ | $FWHM\pm\sigma_{FWHM}$ | $\rm T_{mb}\pm\sigma_{T_{mb}}$
---|---|---|---
C II |  (km s-1) |  (km s-1) | (K)
1 |  0.06 $\pm$ 0.01 |  2.2 $\pm$ 0.03 |  8.28 $\pm$ 0.10
2 |  0.95 $\pm$ 0.02 |  3.0 $\pm$ 0.05 |  7.77 $\pm$ 0.11
3 |  1.0 $\pm$ 0.02 |  2.6 $\pm$ 0.06 |  5.55 $\pm$ 0.10
4 |  1.0 $\pm$ 0.03 |  2.6 $\pm$ 0.08 |  3.36 $\pm$ 0.09
Region | $\rm V_{peak}\pm\sigma_{V_{peak}}$ | $FWHM\pm\sigma_{FWHM}$ | $\rm I_{peak}\pm\sigma_{I_{peak}}$
$\rm HCO^{+}$ |  (km s-1) |  (km s-1) | (Jy/beam)
1 |  0.27 $\pm$ 0.01 |  1.04 $\pm$ 0.03 |  0.082 $\pm$ 0.002
2 |  0.47 $\pm$ 0.05 |  1.96 $\pm$ 0.11 |  0.026 $\pm$ 0.001
3 |  0.37 $\pm$ 0.04 |  1.81 $\pm$ 0.10 |  0.020 $\pm$ 0.001
4 |  0.43 $\pm$ 0.05 |  1.96 $\pm$ 0.12 |  0.021 $\pm$ 0.001
Table 2: Table of the parameters for the Gaussians fitted to the [C II] velocity spectra of IC 59 for each region in Figure 11. Region | $\rm V_{peak}\pm\sigma_{V_{peak}}$ | $FWHM\pm\sigma_{FWHM}$ | $\rm T_{mb}$
---|---|---|---
C II |  (km s-1) |  (km s-1) | (K)
1 |  -0.58 $\pm$ 0.01 |  2.5 $\pm$ 0.03 |  2.6 $\pm$ 0.03
2 |  -1.2 $\pm$ 0.05 |  3.2 $\pm$ 0.11 |  1.3 $\pm$ 0.04
3 |  -0.21 $\pm$ 0.02 |  3.5 $\pm$ 0.05 |  3.1 $\pm$ 0.04
4 |  -0.53 $\pm$ 0.01 |  2.4 $\pm$ 0.03 |  4.3 $\pm$ 0.05
5 |  -0.26 $\pm$ 0.03 |  3.0 $\pm$ 0.06 |  1.9 $\pm$ 0.04

The differences in peak $\rm T_{mb}$ between the two clouds is very obvious in
these tables, with the most intense regions in IC 59 having about the same
peak intensity as the two weakest regions in IC 63. The central velocities
also vary, with IC 63 going from $\sim$0 to 1 km s-1 and IC 59 ranging from
-0.5 to -1.2 km s-1.

Most of the spectra for IC 63 in Figure 10 show a slight, but systematic
asymmetry, with a more gradual rise on the blue side and a steeper drop off on
the red for [C II] and a fairly uniform shape for $\rm HCO^{+}$ (J=1-0).

###  4.4 Velocity Gradients and Magnetic Fields

Figure 12: Central velocity maps of IC 63, top, and IC 59, bottom, with their
respective velocity gradient vectors plotted in black. Magnetic field
polarization vectors are plotted in white on both maps and represent data from
Andersson et al. (2013); Soam et al. (2017) for IC 63 and Soam et al. (2017)
for IC 59. The length of the lines are representative of the magnitude of the
velocity changes for the velocity gradients and the polarization percentages
for the magnetic field polarizations in both maps.

In order to understand any relation between gas velocity and magnetic fields,
we calculated velocity gradient of [C II] emission and compared those with
magnetic field orientations taken from Soam et al. (2017). These gradients are
not absolute velocities, but indicate local accelerations. Gradients were
calculated by taking the difference between the central velocities - estimated
through fitting each pixel’s velocity spectrum with a Gaussian - of the pixels
a specified distance from a central pixel. Only velocity gradients with less
than a 20∘ error in angle are used. Different step distances, 2 vs. 4 pixels,
were used for IC 63 and IC 59 respectively, to best show the dynamics for the
clouds. A larger step size was needed for the gradients in IC 59 due to the
cloud being much larger than IC 63 and the pixel-to-pixel velocity change
being smaller.

Figure 12 shows the local velocity gradient vectors of IC 63 and IC 59
overlaid in black on their respective central velocity maps. Previously
published magnetic field orientations from Andersson et al. (2013); Soam et
al. (2017) for both nebulae, obtained from observations of partially
extinguished starlight from stars located behind the nebula, are plotted with
white line segments. It is hard to draw any firm conclusions on the relative
orientations of the velocity gradients and magnetic field lines. However,
there are some hints of the magnetic fields being perpendicular to the
velocity gradients in both the nebulae. High-resolution polarization
observations in longer wavelengths may shed more light on this investigation.

##  5 Discussion

###  5.1 FUV Radiation Field at IC 63

The FUV radiation field ($\rm G_{0}$) incident on IC 63 from $\gamma$ Cas has
been estimated by various previous studies. Jansen et al. (1995), using the
projected distance between $\gamma$ Cas and IC 63 of 1.3 pc and a distance to
$\gamma$ Cas of 230 pc, found that $\rm G_{0}$ at IC 63 was 650 units of the
Draine field (Draine, 1978). Andrews et al. (2018) used dust emissions in IC
63 instead of spectral emissions from $\gamma$ Cas and found a $\rm G_{0}$ of
150, indicating that the nebula was further away than what Jansen et al.
(1995) assumed as the projected distance. Andersson et al. (2013) proposed
that, based on the angle of the streaming tails in IC 63, the cloud lies
behind $\gamma$ Cas at an angle of 58∘, making the true distance between
$\gamma$ Cas and IC 63 $\sim$2 pc.

Since $\gamma$ Cas is a variable star, comparing the FUV spectra from Code &
Meade (1979), used by Jansen et al. (1995), to a newer spectrum from France et
al. (2005), gives a better idea of what the star may have been like during the
measurements of IC 63 shown by Andrews et al. (2018) as well as systematic
errors in the processes used. It is found that the flux from the France et al.
(2005) spectra is $\sim$.65 $\times$ the flux found in the work by Code &
Meade (1979). Using both of these spectra, the newer parallax to $\gamma$ Cas,
and a distance from $\gamma$ Cas to IC 63 of 2 pc, we find that $\rm G_{0}$ at
IC 63 is 115 and 175 from France et al. (2005) and Code & Meade (1979),
respectively.

###  5.2 FUV Radiation Field at IC 59

Figure 13: Side view model of the IC 59 - $\gamma$ Cas system with geometrical
findings based on $\rm G_{0}$ calculations.

We are unable to estimate a $\rm G_{0}$ for IC 59 in the same way as we did in
Section 5.1 for IC 63, because IC 59 has a mostly vertical orientation in the
plane of the sky which would mitigate any projection effects from an
inclination angle. Using the projected distance from $\gamma$ Cas to IC 59 of
1.5 pc provides a $\rm G_{0}$ that is far higher than the 25 found by Andrews
et al. (2018). There may be a possibility of leakage of factors FUV radiation
field due to clumpiness in the clouds. In order to find the inclination angle
for IC 59, we will assume that the $\rm G_{0}$ from Andrews et al. (2018) and
our methods used in Section 5.1 are correct because our calculated $\rm G_{0}$
for IC 63 agrees, within statistical errors, with the 150 that Andrews et al.
(2018) found.

Utilizing these assumptions, we calculate that a distance of 4.5 pc from
$\gamma$ Cas to IC 59 will produce a $\rm G_{0}$ of 25. This distance
corresponds to an inclination angle of 70∘ with respect to the plane of the
sky. Figure 13 shows the proposed geometry for the IC 59 - $\gamma$ Cas system
as viewed from the side.

###  5.3 Kinetic vs. Magnetic Energy

In IC 63, Figure 12 upper panel, [C II] velocity gradients show a fairly
connected system that moves from velocities of almost 0 km s-1 at the main
clump into the velocities of 1 km s-1 or more in the tails. No obvious
structural correlation could be found when comparing the local velocity
gradients and the magnetic field orientations at IC 63.

In IC 59, the data do not show a connected system (see lower panel of Figure
12). There is slightly more anti-correlation observed between the local
velocity gradients and the magnetic field orientations in IC 59 than what was
observed in IC 63. As described in Soam et al. (2017), this may be related to
the concave shape created by UV radiation on the star-facing side. The field
lines follow this concave shape of the cloud rather than going along the
radiation direction as seen in IC 63

Soam et al. (2017) found the dynamical pressure in IC 63 to be less than the
magnetic pressure, and both were less than the external pressure for IC 63.
Using their magnetic field strength average over the main body of the PDR
(region II in their Figure 1) and an average velocity change in our velocity
gradients, we estimate magnetic and kinetic energy values of $\rm E_{mag}$ =
$\rm 3.4x10^{32}~{}J$ and $\rm E_{kin}$ = $\rm 3.8x10^{42}~{}J$, respectively,
which suggests that the kinetic energy dominates the magnetic energy in IC 63.

###  5.4 Geometry of the System

Figure 14: RGB from upper panel of Figure 2 with smoothed contours of [C II]
integrated intensity below -0.5 km s-1 in white with levels [5,7,10,13] K km
$\rm s^{-1}$ and smoothed H$\alpha$ contours in blue with levels [150,200,300]
pixel counts. The beam size for the SOFIA [C II] observations, white contour,
is plotted in the bottom left corner.

Figure 15: 250 $\micron$ dust map of IC 59 with C [II] integrated intensity
contours at levels [10, 13, 16, 20] K km $\rm s^{-1}$ in white for visual
reference and $\rm{}^{12}CO$ (1-0) contours at levels [1500, 2500, 3300] K km
s-1 in black. Markers are stars from the VILNIUS Survey within 1250 pc with an
$\rm A_{v}$ less than 1 for the yellow circles and greater than 1 for the red
triangles. Beam size of the $\rm{}^{12}CO$ (1-0) contours is shown in black in
bottom left corner.

As we have found that each nebula lies off the plane of the sky due to actual
distances from the stars to the nebula being larger than the projected
distances, we attempted to find whether the nebulae were in front of or behind
$\gamma$ Cas.

For IC 63, the RGB images in Figure 2, the shifting features seen in the
channel map in Figure 6, the PV diagrams in Figure 8, and the velocity
gradients in Figure 12, indicate that the blue shifted [C II] gas, where $\rm
v<\,0\,km\,s^{-1}$, is correlated with the H$\alpha$ emission that is likely
due to photo-evaporation. This correlation is shown in Figure 14, where the
same RGB image as the middle panel of Figure 2, with red as [C II] integrated
intensity, green as $\rm H_{2}$ (1-0) S(1), and blue as H$\alpha$, has been
plotted with H$\alpha$ contours in blue and contours of the [C II] integrated
intensity map of velocities below -0.1 km s-1 are plotted in white. These two
contours correlate well, with the ionized H$\alpha$ sitting just outside the
cloud of the blue shifted velocities of [C II]. Also, as shown in Table 1,
while the [C II] central velocities range from 0 - 1 km s-1 the $\rm HCO^{+}$
(J=1-0) velocities only range from $\sim$ 0.27 - 0.47 km s-1. This suggests
that these two emissions represent the same region and that the gasses in this
region are both being actively ionized and blue shifted while the rest of the
cloud is not. This suggests that IC 63 lies on the far side of $\gamma$ Cas.
Our finding is consistent with findings in Andersson et al. (2013).

Similarly, the RGB images of IC 59 in Figure 3, the channel map in Figure 7,
the PV diagrams in Figure 9, and the velocity gradients in Figure 12, show
that the H$\alpha$ emission does not overlap with the dust or gas emissions.
Because the H$\alpha$ emission in IC 59 does not overlap with the rest of the
dust or gas emissions, we consider a slightly differently approach than for IC
63. The locations that are closest to the H$\alpha$ emission are redshifted in
IC 59, with v $>$ 0 km s-1, indicating that the emission could be coming from
the far side of the cloud. The H$\alpha$ emission coincident with the [C II]
emission would then likely be extincted by the cloud. Figure 15 shows the 250
$\micron$ dust map for IC 59 with [C II] integrated intensity contours in
white and $\rm{}^{12}CO$ (1-0) contours in black. The filled circles and
triangles indicate the locations of stars less than 1.25 kpc distance with
measures of visual extinctions $\rm A_{V}$ (Soam et al., 2021d). Yellow
circles show stars with an $\rm A_{V}$ $<$ 1 mag, while red triangles indicate
stars with an extinction $>$ 1 mag.

The differences in the relative velocities of the high density regions of the
two clouds, along with the opposing relative velocity differences of the
diffuse gas emissions in between the arms of each cloud (Section 4.2.2)
support that the IC 59 and IC 63 are on opposite sides of $\gamma$ Cas. We
therefore predict that IC 59 is on the near side of $\gamma$ Cas.

Our proposed geometric model of the Sh 2-185 system is shown in a simplified
form in Figure 16, where the nebulae are modeled by red, green, and blue
colors representing positive, zero, and negative velocities respectively. The
dashed lines from the star indicate the plane of the sky containing $\gamma$
Cas.

Figure 16: Cartoon model of the $\gamma$ Cas, IC 59, and IC 63 system. Dashed
black lines represent the plane of the sky at the distance of $\gamma$ Cas.

##  6 Summary

We studied the structure and kinematics of PDRs IC 59 and IC 63 associated
with H II region Sh 2-185 using high-resolution SOFIA/upGREAT [C II]
observations combined with archival data on dust emission toward these
nebulae. The case of IC 59 and IC 63 presents an interesting example in which
the feedback from a B0 star is dominated by photevaporation.

  1. 1.

The [C II] emission of IC 63 has two clumps with streaming tails showing a
velocity shift towards positive velocities, moving away from $\gamma$ Cas. IC
59 has a less defined structure, consisting of multiple low density regions
and one large core that is redshifted in comparison to the rest of the cloud,
which otherwise shift from positive to negative velocities when going away
from $\gamma$ Cas.

  2. 2.

A $\rm G_{0}$ of 115-175 was found for IC 63 using a previously determined
inclination angle of 58∘ (Andersson et al., 2013). Utilizing a $\rm G_{0}$ for
IC 59 of 25 (Andrews et al., 2018), a distance of 4.5 pc and inclination angle
of 70∘ could be estimated for the cloud.

  3. 3.

Using [C II] data, we constructed velocity gradients and compared these to
plane-of-the-sky magnetic field orientations. We find an anti-correlation
between the velocity gradient and magnetic fields in IC 59, but no clear
structure in IC 63. The upper limits of our estimations showed that kinetic
energy is larger than the magnetic energy in IC 63. This suggests that kinetic
pressure in this nebula is dominant.

  4. 4.

From our analysis of the kinematics and structure of these clouds, we find
that IC 63 lies behind $\gamma$ Cas and IC 59 lies in front.

##  7 Acknowledgements

Based in part on observations made with the NASA and DLR Stratospheric
Observatory for Infrared Astronomy (SOFIA). SOFIA is jointly operated by the
Universities Space Research Association, Inc. (USRA), under NASA contract
NNA17BF53C, and the Deutsches SOFIA Institut (DSI) under DLR contract 50 OK
0901 to the University of Stuttgart. The GREAT team was an invaluable resource
in obtaining and utilizing the data for this project.

Financial support for this work was provided by NASA through award 05_0052
issued by USRA. A.S. and B-G.A. are supported by National Science Foundation
Grant-1715876. This material is also based upon work supported by the National
Science Foundation under Grant No. 1715060.

This paper makes use of data obtained as part of the INT Photometric H$\alpha$
Survey of the Northern Galactic Plane (IPHAS, www.iphas.org) carried out at
the Isaac Newton Telescope (INT). The INT is operated on the island of La
Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los
Muchachos of the Instituto de Astrofisica de Canarias. All IPHAS data are
processed by the Cambridge Astronomical Survey Unit, at the Institute of
Astronomy in Cambridge. The bandmerged DR2 catalogue was assembled at the
Centre for Astrophysics Research, University of Hertfordshire, supported by
STFC grant ST/J001333/1.

Facility: upGREAT/SOFIA Softwares: Astropy (Astropy Collaboration et al.,
2013), StarNet++, CLASS

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##  Appendix A Thermal Pressure

Thermal pressures of these PDRs can be calculated with:

${\rm\frac{P_{[C\,II]}}{k_{B}}=n_{[C\,II]}T_{ex}}$ (A1)

where $\rm n_{[C\,II]}$ is estimated from column density assuming the
prominent $\rm[C\,II]$ region is a cylinder projected as a rectangle, and we
assume that the [C II] excitation is thermalized. In IC 63 this cylinder is
estimated to have a length of 0.15 pc and radius of 0.05 pc. This gives a one
sigma limit of $\rm P_{[C\,II]}$/$\rm k_{B}$ to be 210.6 K $\rm cm^{-3}$. If
we use $\rm n_{H_{2}}$ = 5 $\pm 2$ $\times$ $\rm 10^{4}$ $\rm cm^{-3}$ (Jansen
et al., 1994) and $\rm T_{kin}$ = 106 $\pm 11$ K (Thi et al., 2009), then we
get a $P_{H_{2}}$/$k_{B}$ of $\rm 53\times 10^{6}$ K $\rm cm^{-3}$. The
$\rm[C\,II]$/$\rm H_{2}$ (1-0) S(1) ratio, using our calculated $\rm
n_{[C\,II]}$ and the $\rm n_{H_{2}}$ from Jansen et al. (1994), is found to be
1.1 $\times$ $\rm 10^{-5}$. This is about a factor of 10 off of the abundance
ratios calculated for the horse head nebula in Bally et al. (2018). This could
be partially explained by a difference in volume estimations between our work
and Jansen et al. (1994). Using equation A1 for [C II] in an area of IC 59
with a length of 0.315 pc and radius of 0.101 pc, the one sigma limit of $\rm
P_{[C\,II]}$/$\rm k_{B}$ is found to be 29.19 K $\rm cm^{-3}$.