eurm10 msam10 1

# Massive Star Formation
in the Galactic Center

Don F.Figer  Rochester Institute of Technology, Rochester, NY, USA

(2006)

###### Abstract

The Galactic center is a hotbed of star formation activity, containing the
most massive star formation site and three of the most massive young star
clusters in the Galaxy. Given such a rich environment, it contains more stars
with initial masses above 100 M⊙ than anywhere else in the Galaxy. This review
concerns the young stellar population in the Galactic center, as it relates to
massive star formation in the region. The sample includes stars in the three
massive stellar clusters, the population of younger stars in the present sites
of star formation, the stars surrounding the central black hole, and the bulk
of the stars in the field population. The fossil record in the Galactic center
suggests that the recently formed massive stars there are present-day examples
of similar populations that must have been formed through star formation
episodes stretching back to the time period when the Galaxy was forming.

††volume: 1

##  1 Introduction

The Galactic center (GC) is an exceptional region for testing massive star
formation and evolution models. It contains 10% of the present star formation
activity in the Galaxy, yet fills only a tiny fraction of a percent of the
volume in the Galactic disk111For the purposes of this review, the GC refers
to a cylindrical volume with radius of $\approx$500 pc and thickness of
$\approx$60 pc that is centered on the Galactic nucleus and is coincident with
a region of increased dust and gas density, often referred to as the “Central
Molecular Zone” (Serabyn & Morris, 1996).. The initial conditions for star
formation in the GC are unique in the Galaxy. The molecular clouds in the
region are extraordinarly dense, under high thermal pressure, and are subject
to a strong gravitational tidal field. Morris (1993) argue that these
conditions may favor the preferential formation of high mass stars. Being the
closest galactic nucleus, the GC gives us an opportunity to observe processes
that potentially have wide applicability in other galaxies, both in their
centers and in the interaction regions of merging galaxies. Finally, the GC
may be the richest site of certain exotic processes and objects in the Galaxy,
i.e. runaway stellar mergers leading to intermediate mass black holes and
stellar rejuvination through atmospheric stripping, to name a few.

This review is primarily concerned with massive star formation in the region.
For thorough reviews on a variety of topics concerning the Galactic center,
see Genzel & Townes (1987), Genzel et al. (1994), Morris & Serabyn (1996), and
Eckart et al. (2005).

##  2 The Galactic center environment and star formation

The star formation efficiency in the GC appears to be high. Plotting the
surface star formation rate ($\Sigma_{\rm SFR}\sim$5 M⊙ yr-1 pc-2) versus
surface gas density ($\Sigma_{H_{2}}\sim$400 M⊙ pc-2) in a “Schmidt plot”
suggests an efficiency of nearly 100%, comparable to that of the most intense
infrared circumnuclear starbursts in other galaxies and a factor of twenty
higher than in typical galaxies (see Figure 7 in Kennicutt, 1998). It is also
higher than that elsewhere in the Galaxy; commensurately, stars in the GC emit
about 5-10% of the Galaxy’s ionizing radiation and infrared luminosity.

Morris & Serabyn (1996) review the content and conditions of the interstellar
medium in the “Central Molecular Zone” (CMZ), noting that the molecular clouds
in the region are extraordinarily dense ($n>10^{4}~{}cm^{-3}$) and warm
($T\sim 70~{}K$) with respect to those found in the disk of the Galaxy. Stark
et al. (1989) argue that the density and internal velocities of clouds in the
GC are a direct result of the strong tidal fields in the region, i.e. only the
dense survive. Serabyn & Morris (1996) argue that the inexorable inflow of
molecular material from further out in the Galaxy powers continuous and robust
star formation activity in the region.

It is still unclear how magnetic field strength affects star formation. If it
does matter, then the GC might be expected to reveal such effects. The
strength of the magnetic field in the GC has been estimated through far
infrared polarized light from aligned dust grains (Hildebrand et al., 1993;
Chuss et al., 2003) and Zeeman splitting of the OH molecule (Plante, Lo, &
Crutcher, 1995). In both cases, the field is inferred to be of milliGuass
strength. However, Uchida & Guesten (1995) argue strongly that these strengths
are localized to bundles that delineate the extraordinary non-thermal
filaments in the region (Yusef-Zadeh & Morris, 1987), and are not
representative of the field strength that is pervasive in the region. If this
is correct, then the fields inside GC molecular clouds may not be so strong
versus those inside disk clouds ($B\sim 3~{}\mu$G).

Metals in molecular clouds can provide cooling that aids protostellar
collapse, but they also create opacity to the UV flux, winds, and bipolar
outflows that emanate from newly formed stars. Measurements of metallicty in
the Galactic center span a range of solar, observed in stars (Ramírez et al.,
2000; Carr, Sellgren, & Balachandran, 2000; Najarro et al., 2004), to twice
solar, observed in the gas phase (Shields & Ferland, 1994), to four times
solar, observed through x-ray emission near the very center (Maeda et al.,
2002). The errors from the stellar measurements are the smallest and suggest
that stars in the GC are formed from material with roughly solar abundances.

##  3 Present-day star formation in the GC

Present-day star formation in the GC is somewhat subdued compared to the
episodes that produced the massive clusters we now see. A dozen or so ultra-
compact HII regions are distributed throughout the central 50 pc, each
containing one or a few O-stars still embedded in their natal environs. Yusef-
Zadeh & Morris (1987) identify most of these sources in radio continuum
observations (see Figure 1). Zhao et al. (1993) and Goss et al. (1985) infer
lyman-continuum fluxes that are comparable to that expected from a single O7V
star in each of the H1-H5 and A-D UCHII regions. Cotera et al. (1999) find
that several of the recently formed stars in these regions have broken out of
their dust shroud, revealing spectra of young massive stars; see also Figer et
al. (1994) and Muno et al. (2006) for additional examples.

Figure 1: Radio emission from the GC region at 6 cm, adapted in Figure 1 by
Cotera et al. (1999) from Yusef-Zadeh & Morris (1987). The star symbols
represent the three massive clusters. Hot stars in the Quintuplet and Arches
clusters ionize gas on the surfaces of nearby molecular clouds to produce the
radio emission in the “Sickle” and “G0.10+0.02/E1/E2 Filaments,” respectively.
The radio emission near the Galactic center is due to a combination of thermal
and non-thermal emission. The “H1-8” and “A-D” regions are ultra-compact HII
regions surrounding recently formed stars.

A bit further from the GC, the Sgr B2 molecular cloud harbors a massive star
cluster in the making and is home to the most intense present-day star
formation site in the Galaxy (Gaume et al., 1995; de Pree et al., 1995;
McGrath, Goss, & De Pree, 2004; Takagi, Murakami, & Koyama, 2002; de Vicente
et al., 2000; Liu & Snyder, 1999; Garay & Lizano, 1999; de Pree et al., 1996).
Within the next few Myr, this activity should produce a star cluster that is
comparable in mass to the Arches cluster (see Figure 2). Sato et al. (2000)
note evidence in support of a cloud-cloud collision as the origin for the
intense star formation in Sgr B2; these include velocity gradients, magnetic
field morphology, shock-enhanced molecular emission, shock-induced molecular
evaporation from dust grains, and distinctly different densities of certain
molecular species throughout the cloud.

Figure 2: Figure 3 from McGrath, Goss, & De Pree (2004) showing H2O and OH
masers overplotted on 7 mm contours for a small portion of the Sgr B2 cloud.
The activity in this region is typical of that found near the fifty or so
ultra-compact HII regions in Sgr B2.

##  4 Continuous star formation in the GC

There is ample evidence for persistent star formation in the GC in the form of
upper-tip asymptotic giant branch stars distributed throughout the region
(Lebofsky & Rieke, 1987; Narayanan, Gould, & Depoy, 1996; Frogel, Tiede, &
Kuchinski, 1999; Sjouwerman et al., 1999). Figure 3 shows a plot for some of
these stars, based on spectroscopic data, overlaid with intermediate age model
isochrones (Blum et al., 2003). Note that the giants and supergiants in this
plot require ages that span a few Myr to a few Gyr.

One comes to similar conclusions by analyzing photometry of the field
population in the GC. Figer et al. (2004) use observed luminosity functions to
determine that the star formation rate has been roughly constant for the
lifetime of the Galaxy in the GC, similar to the suggestion in Serabyn &
Morris (1996) based on the sharp increase in unresolved infrared light towards
the center and a mass-budget argument. Figure 4 shows model and observed
luminosity functions (right) for various star formation scenarios (left) over
the lifetime of the Galaxy, assuming a Salpeter IMF (Salpeter, 1955) for
masses above 10 M⊙, and a flat slope below this mass. The observations were
obtained with HST/NICMOS and have been corrected for incompleteness. The
“burst” models (panels 1, 2, 4, and 5) produce unrealistic ratios of bright to
faint stars in the luminosity functions, especially for the red clump near a
dereddened K-band magnitude of 12. The continuous star formation model (panel
3) best fits the data.

Figure 3: Estimates of absolute magnitude versus temperature for stars in the
GC from Blum et al. (2003). The lines correspond to model isochrones having
ages of 10 Myr, 100 Myr, 1 Gyr, 5 Gyr, and 12 Gyr. The supergiants (above the
horizontal line) are descendant from stars having M$\approx$15-25 M⊙, whereas
fainter stars are descendant from lower mass main sequence stars having a few
to 15 M⊙. The presence of these stars in the GC demonstrates intermediate age
star formation of massive stars.
Figure 4: A figure adapted from Figer et al. (2004) showing various star
formation scenarios (left), and resultant model luminosity functions (right,
thick) compared to observed luminosity functions (right, thin) in the GC. The
models assume a Salpeter IMF slope, an elevated lower-mass turnover of 10 M⊙,
and are additionally constrained to produce 2(108) M⊙ in stars within the
region. The observations have been corrected for incompleteness. The third
panels from the top, i.e. continuous star formation, best fit the data. The
observed turn-down at the faint end appears to be real and is only well fit
only by assuming a very high lower mass turnover.

##  5 Properties of the Three Massive Clusters

The majority of recent star formation activity in the GC over the past 10 Myr
produced three massive clusters: the Central cluster, the Arches cluster, and
the Quintuplet cluster. The following sections describe the stellar content in
the clusters and the resultant implications for star formation in the region.
They closely follow recent reviews (Figer et al., 1999a; Figer, 2003, 2004),
with updates, as summarized in Table 1.

Table 1: Properties of massive clusters in the Galactic Center

000“M1” is the total cluster mass in observed stars. “M2” is the total cluster
mass in all stars extrapolated down to a lower-mass cutoff of 1 M⊙, assuming a
Salpeter IMF slope and an upper mass cutoff of 120 M⊙(unless otherwise noted)
“Radius” gives the average projected separation from the centroid position.
“$\rho 1$” is M1 divided by the volume. “$\rho 2$” is M2 divided by the
volume. In either case, this is probably closer to the central density than
the average density because the mass is for the whole cluster while the radius
is the average projected radius. “Age” is the assumed age for the cluster.
“Luminosity” gives the total measured luminosity for observed stars. “Q” is
the estimated Lyman continuum flux emitted by the cluster.

| Log(M1) | Log(M2) | Radius | Log($\rho 1$) | Log($\rho 2$) | Age | Log(L) | Log(Q)
---|---|---|---|---|---|---|---|---
Cluster | M⊙ | M⊙ | pc |  M⊙ pc-3 |  M⊙ pc-3 | Myr | L⊙ | s-1
Quintuplet | 3.0 | 3.8 | 1.0 | 2.4 | 3.2 | 3$-$6 | 7.5 | 50.9
Arches111Mass estimates have been made based upon the number of stars having Minitial$>$20 M⊙ given in Figer et al. (1999b) and the mass function slope in Stolte et al. (2003). The age, luminosity and ionizing flux are from Figer et al. (2002). | 4.1 | 4.1 | 0.19 | 5.6 | 5.6 | 2$-$3 | 8.0 | 51.0
Center222Krabbe et al. (1995). The mass, “M2” has been estimated by assuming that a total 103.5 stars have been formed. The age spans a range covering an initial starburst, followed by an exponential decay in the star formation rate. | 3.0 | 4.0 | 0.23 | 4.6 | 5.6 | 3$-$7 | 7.3 | 50.5

The three clusters are similar in many respects, as they are all young and
contain $\gtrsim$104 M⊙ in stars. They have very high central stellar mass
densities, up to nearly 106 M⊙ pc-3, exceeding central densities in most
globular clusters. They have luminosities of 107-8 L⊙, and are responsible for
heating nearby molecular clouds. They also generate 1050-51 ionizing photons
per second, enough to account for nearby giant HII regions. The primary
difference between the clusters is likely to be age, where the Quintuplet and
Central clusters are about twice the age of the Arches cluster. In addition,
the Central cluster is unique for its population of evolved massive stars that
have broad and strong helium emission lines (Krabbe et al., 1991, and
referenes therein). While the Quintuplet cluster has a few similar stars
(Geballe et al., 1994; Figer et al., 1999a), the Central cluster has far more
as a fraction of its total young stellar population (Paumard et al., 2006).

Table 2 summarizes the massive stellar content of the clusters.

Table 2: Massive Stars in the Galactic Center Clusters | Age (Myr) | O | LBV | WN | WC | RSG |  References
---|---|---|---|---|---|---|---
Quintuplet | 4 | 100 | 2 | 6 | 11 | 1 |  Figer et al. (1999a); Geballe, Najarro, & Figer (2000); Homeier et al. (2003)
Arches | 2 | 160 | 0 |  $\mathrel{\hbox{\hbox to0.0pt{\hbox{\lower 4.0pt\hbox{$\sim$}}\hss}\hbox{$>$}}}$6 | 0 | 0 |  Figer et al. (2002)
Center |  4$-$7 | 100 |  $\mathrel{\hbox{\hbox to0.0pt{\hbox{\lower 4.0pt\hbox{$\sim$}}\hss}\hbox{$>$}}}$1 |  $\mathrel{\hbox{\hbox to0.0pt{\hbox{\lower 4.0pt\hbox{$\sim$}}\hss}\hbox{$>$}}}$18 |  $\mathrel{\hbox{\hbox to0.0pt{\hbox{\lower 4.0pt\hbox{$\sim$}}\hss}\hbox{$>$}}}$12 | 3 |  Paumard et al. (2006)
Total |  | 360 |  $\mathrel{\hbox{\hbox to0.0pt{\hbox{\lower 4.0pt\hbox{$\sim$}}\hss}\hbox{$>$}}}$3 |  $\mathrel{\hbox{\hbox to0.0pt{\hbox{\lower 4.0pt\hbox{$\sim$}}\hss}\hbox{$>$}}}$29 |  $\mathrel{\hbox{\hbox to0.0pt{\hbox{\lower 4.0pt\hbox{$\sim$}}\hss}\hbox{$>$}}}$23 | 4 |

###  5.1 Central cluster

The Central cluster contains many massive stars that have recently formed in
the past 10 Myr (Becklin et al., 1978; Rieke, Telesco, & Harper, 1978;
Lebofsky, Rieke, & Tokunaga, 1982; Forrest et al., 1987; Allen, Hyland, &
Hillier, 1990; Krabbe et al., 1991; Najarro et al., 1994; Krabbe et al., 1995;
Najarro, 1995; Libonate et al., 1995; Blum, Depoy, & Sellgren, 1995a; Blum,
Sellgren, & Depoy, 1995b; Genzel et al., 1996; Tamblyn et al., 1996; Najarro
et al., 1997). In all, there are now known to be at least 80 massive stars in
the Central cluster (Eisenhauer et al., 2005; Paumard et al., 2006), including
$\approx$50 OB stars on the main sequence and 30 more evolved massive stars
(see Figure 5). These young stars appear to be confined to two disks (Genzel
et al., 2003; Levin & Beloborodov, 2003; Paumard et al., 2006; Tanner et al.,
2006; Beloborodov et al., 2006). There is also a tight collection of a dozen
or so B stars (the “s” stars) in the central arcsecond, highlighted in the
small box in the figure. The formation of so many massive stars in the central
parsec remains as much a mystery now as it was at the time of the first
infrared observations of the region. Most recently, this topic has largely
been supplanted by the even more improbable notion that star formation can
occur within a few thousand AU of the supermassive black hole, an idea that
will be addressed in Section 7. See Alexander (2005) for a thorough review of
the “s” stars and Paumard et al. (2006) for a review of the young population
in the Central cluster.

Figure 5: K-band image of the Central cluster obtained with NAOS/CONICA from
Schödel et al. (2006). The 100 or so brightest stars in the image are evolved
descendants from main sequence O-stars. The central box highlights the “s”
stars that are presumably young and massive (Minitial$\approx$20 M⊙).

###  5.2 Arches cluster

The Arches cluster is unique in the Galaxy for its combination of
extraordinarily high mass, M$\approx$104 M⊙, and relatively young age,
$\tau=2~{}Myr$ (Figer et al., 2002). Being so young and massive, it contains
the richest collection of O-stars and WNL stars in any cluster in the Galaxy
(Cotera et al., 1996; Serabyn, Shupe, & Figer, 1998; Figer et al., 1999b; Blum
et al., 2001; Figer et al., 2002). It is ideally suited for testing theories
that predict the shape of the IMF up to the highest stellar masses formed (see
Section 6).

The cluster is prominent in a broad range of observations. Figure 6 shows an
HST/NICMOS image of the cluster – the majority of the bright stars in the
image have masses greater than 20 M⊙. The most massive dozen or so members of
the cluster have strong emission lines at infrared wavelengths (Harris et al.,
1994; Nagata et al., 1995; Cotera, 1995; Figer, 1995; Cotera et al., 1996;
Figer et al., 1999b; Blum et al., 2001; Figer et al., 2002). These lines are
produced in strong stellar winds that are also detected at radio wavelengths
(Lang, Goss, & Rodríguez, 2001; Yusef-Zadeh et al., 2003; Lang et al., 2005;
Figer et al., 2002), and x-ray wavelengths (Yusef-Zadeh et al., 2002;
Rockefeller et al., 2005; Wang, Dong, & Lang, 2006).

Figure 6: F205W image of the Arches cluster obtained by Figer et al. (2002)
using HST/NICMOS. The brightest dozen or so stars in the cluster have
Minitial$\gtrsim$100 M⊙, and there are $\approx$160 O-stars in the cluster.
The diameter is $\approx$1 lyr, making the cluster the densest in the Galaxy
with $\rho>10^{5}~{}\hbox{\it M${}_{\odot}$}~{}pc^{-3}$.

###  5.3 Quintuplet cluster

The Quintuplet cluster was originally noted for its five very bright stars,
the Quintuplet Proper Members (QPMs) (Glass, Moneti, & Moorwood, 1990; Okuda
et al., 1990; Nagata et al., 1990). Subsequently, a number of groups
identified over 30 stars evolved from massive main sequence stars (Geballe et
al., 1994; Figer, McLean, & Morris, 1995; Timmermann et al., 1996; Figer et
al., 1999a). Given the spectral types of the massive stars identified in the
cluster, it appears that the Quintuplet cluster is $\approx$4 Myr old and had
an initial mass of $>$104 M⊙ (Figer et al., 1999a). An accounting of the
ionizing flux produced by the massive stars in the cluster conclusively
demonstrates that the cluster heats and ionizes the nearby “Sickle” HII region
(see Figure 1). The Quintuplet is most similar to Westerlund 1 in mass, age,
and spectral content (Clark et al., 2005; Negueruela & Clark, 2005; Skinner et
al., 2006; Groh et al., 2006; Crowther et al., 2006).

Of particular interest in the cluster, the QPMs are very bright at infrared
wavelengths, mK $\approx$ 6 to 9, and have color temperatures between
$\approx$ 600 to 1,000 K. They are luminous, L$\approx$105 L⊙, yet
spectroscopically featureless, making their spectral classification ambiguous.
Figer, Morris, & McLean (1996), Figer et al. (1999a), and Moneti et al. (2001)
argue that these objects are not protostars, OH/IR stars, or protostellar OB
stars. Instead, they claim that these stars are dust-enshrouded WCL stars
(DWCLs), similar to other dusty Galactic WC stars (Williams, van der Hucht, &
The, 1987), i.e. WR 104 (Tuthill, Monnier, & Danchi, 1999) and WR 98A
(Monnier, Tuthill, & Danchi, 1999). Chiar et al. (2003) tentatively identify a
weak spectroscopic feature at 6.2 $\mu$m that they attribute to carbon,
further supporting the hypothesis that these stars are indeed DWCLs. The stars
have also been detected at x-ray wavelengths (Law & Yusef-Zadeh, 2004), and at
radio wavelengths (Lang et al., 1999, 2005).

Recently, Tuthill et al. (2006) convincingly show that the QPMs are indeed
dusty WC stars. Figure 8 shows data that reveal the pinwheel nature of their
infrared emission, characteristic of binary systems containing WCL plus an OB
star (Tuthill, Monnier, & Danchi, 1999; Monnier, Tuthill, & Danchi, 1999).
This identification raises intruiging questions concerning massive star
formation and evolution. With their identifications, it becomes clear that
every WC star in the Quintuplet is dusty, and presumably binary. There are two
possible explanations for this result. Either the binary fraction for massive
stars is extremely high (Mason et al., 1998; Nelan et al., 2004), or only
binary massive stars evolve through the WCL phase (van der Hucht, 2001).

The Quintuplet cluster also contains two Luminous Blue Variables, the Pistol
star (Harris et al., 1994; Figer et al., 1998, 1999c), and FMM362 (Figer et
al., 1999a; Geballe, Najarro, & Figer, 2000). Both stars are extraordinarily
luminous (L$>$106 L⊙), yet relatively cool (T$\approx$104 K), placing them in
the “forbidden zone” of the Hertzsprung-Russell Diagram, above the Humphreys-
Davidson limit (Humphreys & Davidson, 1994). The Pistol star is particularly
intriguing, in that it is surrounded by one of the most massive (10 M⊙)
circumstellar ejecta in the Galaxy (see Figure 7; Figer et al., 1999c; Smith,
2006). Both stars are spectroscopically (Figer et al., 1999a) and
photometrically variable (Glass et al., 2001). They present difficulties for
stellar evolution and formation models. Their inferred initial masses are
$>$100 M⊙, yet such stars should have already gone supernova in a cluster that
is so old, as evidenced by the existence of WC stars (Figer et al., 1999a) and
the red supergiant, q7 (Moneti, Glass, & Moorwood, 1994; Ramírez et al.,
2000). Figer & Kim (2002) and Freitag, Rasio, & Baumgardt (2006) argue that
stellar mergers might explain the youthful appearance of these stars.
Alternatively, these stars might be binary, although no evidence has been
found to support this assertion. Note that in a similar case, LBV1806$-$20 is
also surrounded by a relatively evolved cluster (Eikenberry et al., 2004;
Figer et al., 2005), yet it does appear to be binary (Figer, Najarro, &
Kudritzki, 2004).

Figure 7: Paschen-$\alpha$ image of the region surrounding the Pistol star
from Figer et al. (1999c). North is to the upper right, and east is to the
upper left. The Pistol star ejected $\approx$10 M⊙ of material approximately
6,000 yr ago to form what now appears to be a circumstellar nebula that is
ionized by two WC stars to the north of the nebula. Moneti et al. (2001) use
ISO data to show that the nebula is filled with dust that is heated by the
Pistol star.
Figure 8: Tuthill et al. (2006) find that the Quintuplet Proper Members are
dusty Wolf-Rayet stars in binary systems with OB companions. The insets in
this illustration show high-resolution infrared imaging data for two
Quintuplet stars, overlaid on the HST/NICMOS image from Figer et al. (1999b).
All of the Quintuplet WC stars are dusty, suggesting that they are binary.

##  6 The initial mass function in the Galactic center

The IMF in the Galactic center has primarily been estimated through
observations of the Arches cluster (Figer et al., 1999b; Stolte et al., 2003),
although there have been several attempts to extract such information through
observations of the Central cluster (Genzel et al., 2003; Nayakshin & Sunyaev,
2005; Paumard et al., 2006) and the background population in the region (Figer
et al., 2004). These studies suggest an IMF slope that is flatter than the
Salpeter value.

###  6.1 The slope

Figer et al. (1999b) and Stolte et al. (2003) estimate a relatively flat IMF
slope in the Arches cluster (see Figure 9). Portegies Zwart et al. (2002)
interpret the data to indicate an initial slope that is consistent with the
Salpeter value, and a present-day slope that has been flattened due to
dynamical evolution. Performing a similar analysis, Kim et al. (2000) arrive
at the opposite conclusion – that the IMF truly was relatively flat. The
primary difficulty in relating the present-day mass function to the initial
mass function is the fact that n-body interactions operate on relatively short
timescales to segregate the highest stellar masses toward the center of the
cluster and to eject the lowest stellar masses out of the cluster. Most
analysis is needed to resolve this issue.

###  6.2 Upper mass cutoff

The Arches cluster is the only cluster in the Galaxy that can be used to
directly probe an upper mass cutoff. It is massive enough to expect stars at
least as massive as 400 M⊙, young enough for its most massive members to still
be visible, old enough to have broken out of its natal molecular cloud, close
enough, and at a well-established distance, for us to discern its individual
stars (Figer, 2005). There appears to be an absence of stars with initial
masses greater than 130 M⊙ in the cluster, where the typical mass function
predicts 18 (see Figure 9). Figer (2005) therefore claim a firm upper mass
limit of 150 M⊙. There is additional support for such a cutoff in other
environments (Weidner & Kroupa, 2004; Oey & Clarke, 2005; Koen, 2006; Weidner
& Kroupa, 2006).

Figure 9: Figer (2005) find an apparent upper-mass cutoff to the IMF in the
Arches cluster. Magnitudes are transformed into initial mass by assuming the
Geneva models for $\tau_{\rm age}$=2 Myr, solar metallicity, and the canonical
mass-loss rates. Error bars indicate uncertainty from Poisson statistics. Two
power-law mass functions are drawn through the average of the upper four mass
bins, one having a slope of $-$0.90, as measured from the data, and another
having the Salpeter slope of $-$1.35. Both suggest a dramatic deficit of stars
with Minitial$>$130 M⊙, i.e. 33 or 18 are missing, respectively. These slopes
would further suggest a single star with very large initial mass (MMAX). The
analysis suggests that the probability of there not being an upper-mass cutoff
is $\approx$ 10-8.

###  6.3 Lower mass rollover

Morris (1993) argue for an elevated lower mass rollover in the GC based on the
environmental conditions therein, and only recently have observations been
deep enough to address this claim. Stolte et al. (2005) claim observational
evidence for an elevated cutoff around 6 M⊙ in the Arches cluster; however, in
that case, confusion and incompleteness are serious problems. In addition,
even if the apparent turn-down is a real indication of the initial cluster
population, the lack of low mass stars might result from their ejection
through n-body interactions (Kim et al., 2000; Portegies Zwart et al., 2002).
Field observations should not suffer from such an effect, as the field should
be the repository for low mass stars ejected from massive clusters in the GC.
Figure 4 reveals a turn-down in the observed luminosity function of the field
in the GC at a dereddened K-band magnitude greater than 16. This appears to
not be a feature of incompleteness, as the data are greater than 50% complete
at these magnitudes (Figer et al., 1999b). A more convincing argument, based
on this type of data, will await even deeper observations (Kim et al., 2006).

##  7 The “s” stars

Figure 5 shows a dense collection of about a dozen stars within 1 arcsecond
(0.04 pc) of Sgr A* (Genzel et al., 1997; Ghez et al., 1998, 2000; Eckart et
al., 2002; Schödel et al., 2002; Ghez et al., 2003; Schödel et al., 2003; Ghez
et al., 2005). This cluster stands out for its high stellar density, even
compared to the already dense field population in the GC. Schödel et al.
(2003) and Ghez et al. (2005) (and refereces therein) have tracked the proper
motions of the “s” stars, finding that they are consistent with closed orbits
surrounding a massive, and dark, object having M$\approx 2-4(10^{6})$ M⊙,
consistent with previous claims based on other methods (Lynden-Bell & Rees,
1971; Lacy et al., 1980; Serabyn & Lacy, 1985; Genzel & Townes, 1987; Sellgren
et al., 1987; Rieke & Rieke, 1988; McGinn et al., 1989; Lacy, Achtermann, &
Serabyn, 1991; Lindqvist, Habing, & Winnberg, 1992; Haller et al., 1996). The
orbital parameters for these stars are well determined, as seen in Figure 10
(left), and they require the existence of a supermassive black hole in the
Galactic center. While these stars are useful as gravitational test particles,
they are also interesting in their own right, as they have inferred
luminosities and temperatures that are similar to those of young and massive
stars (Genzel et al., 1997; Eckart et al., 1999; Figer et al., 2000; Ghez et
al., 2003; Eisenhauer et al., 2003, 2005; Paumard et al., 2006). Figure 10
(right) shows the absorption lines that suggest relatively high temperatures.

Table 3: Chronologically sorted list of references that explore hypotheses on the origin of the “s” stars. Some of the references primarily concern the other young stars in the central parsec and are included in the table because they propose ideas that may relate to the origins of the “s” stars. Contributions to this table have been made by Tal Alexander (priv. communication).  Reference |  Description
---|---
Lacy, Townes, & Hollenbach (1982) |  tidal disruption of red giants
Morris (1993) |  compact objects surrounded by material from red giant envelopes (“Thorne-Zytkow objects”)
Davies et al. (1998) |  red giant envelope stripping through n-body interactions
Alexander (1999) |  red giant envelope stripping through dwarf-giant interactions
Bailey & Davies (1999) |  colliding red giants
Morris, Ghez, & Becklin (1999) |  duty cycle of formation from infalling CND clouds and evaporation of gas reservoir by accretion and star-formation light
Gerhard (2001) |  decaying massive cluster
Alexander & Morris (2003) |  tidal heating of stellar envelopes to form “Squeezars”
Genzel et al. (2003) |  stellar rejuvination through red giant mergers
Gould & Quillen (2003) |  “exchange reaction” between massive-star binary and massive black hole
Hansen & Milosavljević (2003) |  stars captured by inspiraling intermediate mass black hole
Levin & Beloborodov (2003) |  formation in nearby gas disk
McMillan & Portegies Zwart (2003); Kim, Figer, & Morris (2004) |  inward migration of young cluster with IMBH
Portegies Zwart, McMillan, & Gerhard (2003); Kim & Morris (2003) |  inward migration of young cluster
Alexander & Livio (2004) |  orbital capture of young stars by MBH-SBH binaries
Milosavljević & Loeb (2004) |  formation in molecular disk
Davies & King (2005) |  tidal strippng of red giant (AGB) stars (but see critique in Goodman & Paczynski, 2005)
Gürkan & Rasio (2005) |  decaying cluster with formation of an IMBH
Haislip & Youdin (2005) |  formation in disk and orbital relaxation.(but see critique in Goodman & Paczynski, 2005)
Levin, Wu, & Thommes (2005) |  dynamical interactions with sinking IMBH
Nayakshin & Sunyaev (2005) |  _in-situ_ formation within central parsec
Nayakshin & Cuadra (2005) |  formation in a fragmenting star disk
Subr & Karas (2005) |  formation in disk and accelerated orbital relaxation
Berukoff & Hansen (2006) |  cluster inspiral and n-body interactions
Hopman & Alexander (2006) |  resonant relaxation of orbits
Freitag, Amaro-Seoane, & Kalogera (2006) |  mass segregation through interactions with compact remnants
Levin (2006) |  star formation in fragmenting disk
Perets, Hopman, & Alexander (2006) |  exchange reactions between massive star binaries induced by efficient relaxation by massive perturbers
Dray, King, & Davies (2006) |  tidal strippng of red giant (AGB) stars (but see critique in Figer et al., 2000)

Oddly, the increased density of the young stars in the central arcsecond is
not matched by the density distribution of old stars. Indeed, there is a
curious absence of late-type stars in the central few arcseconds, as evidenced
by a lack of stars with strong CO absorption in their K-band spectra (Lacy,
Townes, & Hollenbach, 1982; Phinney, 1989; Sellgren et al., 1990; Haller et
al., 1996; Genzel et al., 1996, 2003). This dearth of old stars represents a
true “hole” in three dimensional space, and not just a projection effect. Even
the late-type stars that are projected on to the central parsec generally have
relatively low velocities, suggesting dynamical evidence that the region
nearest to the black hole lacks old stars (Figer et al., 2003).

The existence of such massive and young stars in the central arcseconds is
puzzling, although it is perhaps only an extension of the original problem in
understanding the origins of the young stars identified in the central parsec
over 20 years ago. Table 3 gives a list of recent papers regarding the origin
of the “s” stars. While there are over 30 papers listed in this table, they
can be reduced to a few basic ideas. One class of ideas considers the “s”
stars as truly young. In this case, the “origin” of the “s” stars is often
reduced to the case of massive star formation in the Galactic center region
and transportation of the products to the central arcsecond. The other class
regards the “s” stars as old stars that only appear to be young, i.e. via
atmospheric stripping, merging, or heating. Both classes require new
mechanisms that would be unique to the GC, and they both have considerable
weaknesses. For example, Figer et al. (2000) argue that stripped red giants
would not be as bright as the “s” stars (see Dray, King, & Davies, 2006, for
detailed confirmation). See Alexander (2005) for a more thorough discussion of
the strengths and weaknesses of these ideas.

If the “s” stars are truly young, then that would require massive clumps to
form OB stars (Minitial$\gtrsim$20 M⊙). In addition, the clumps would have to
form from very high density material in order for them to be stable against
tidal disruption. Assuming that the stars formed as far away from the
supermassive black hole as possible, while still permitting dynamical friction
to transport them into the central arcsecond during their lifetimes, then the
required densities must be $>$10${}^{11}~{}cm^{-3}$ (Figer et al., 2000).

The average molecular cloud density in the GC is about five orders of
magnitude less, so highly compressive events might be required to achieve the
necessary densities. Alternatively, the required densities can be reduced if
the stars are gravitationally bound to significant mass, i.e. a surrounding
stellar cluster. Indeed, Gerhard (2001), Portegies Zwart, McMillan, & Gerhard
(2003), and Kim & Morris (2003) showed that particularly massive clusters
could form tens of parsecs outside of the center and be delivered into the
central parsec in just a few million years. The efficiency of this method is
improved with the presence of an intermediate black hole in the cluster
(McMillan & Portegies Zwart, 2003; Kim, Figer, & Morris, 2004). It is key in
any of these cluster transport models that the host system have extremely high
densities of $>$106 M⊙ pc-3, comparable to the highest estimated central
density of the Arches cluster after core collapse (Kim & Morris, 2003).
Detailed n-body simulations suggest that while these ideas may be relevant for
the origins of the young stars in the central parsec, it is unlikely that they
could explain the existence of the “s” stars in the central arcsecond.

Figure 10: (left) Figure 2 in Ghez et al. (2005) and (middle) Figure 6 in
Schödel et al. (2003), stretched to the same scale. Both figures fit similar
model orbits through separate proper motion data sets for the “s” stars.
(right) Paumard et al. (2006) find that one of the “s” stars, S2, has a K-band
spectrum that is similar to those of OB stars in the central parsec (see also
Ghez et al., 2003).

##  8 Comparisons to other massive star populations in Galaxy

There are relatively few clusters in the Galaxy with as many massive stars as
in the GC clusters. NGC3603 has about a factor of two less mass than each of
the GC clusters (Moffat et al., 2002); whereas, W1 has at least a factor of
two greater mass (Clark et al., 2005; Negueruela & Clark, 2005; Skinner et
al., 2006; Groh et al., 2006). The next nearest similarly massive cluster is
R136 in the LMC (Massey & Hunter, 1998). All of these clusters, and the GC
clusters, appear to have IMF slopes that are consistent with the Salpter value
(or slightly flatter) and are young enough to still possess a significant
massive star population. It is remarkable to note that these massive clusters
appear quite similar in stellar content, whether in the Galactic disk, the GC,
or even the lower metallicity environment of the LMC. Evidently, the star
formation processes, and natal environments, that gave birth to these clusters
must be similar enough to produce clusters that are virtually
indistinguishable.

There are probably more massive clusters yet to be found in the Galaxy. The
limited sample of known massive clusters is a direct result of extinction, as
most star formation sites in the Galaxy are obscurred by dust at optical
wavelengths. While infrared observations have been available for over 30
years, they have not provided the necessary spatial resolution, nor survey
coverage, needed to probe the Galactic disk for massive clusters. Recently, a
number of groups have begun identifying candidate massive star clusters using
near-infrared surveys with arcsecond resolution (Bica et al., 2003; Dutra et
al., 2003; Mercer et al., 2005). Indeed, these surveys have already yielded a
cluster with approximate initial mass of 20,000 to 40,000 M⊙ (Figer et al.,
2006), and one would expect more to be discovered from them.

The present-day sites of massive star formation in the Galaxy have been known
for some time through radio and far-infrared observations, as their hottest
members ionize and heat nearby gas in molecular clouds. As one of many
examples, consider W49 which is the next most massive star formation site in
the Galaxy compared to Sgr B2 (Homeier & Alves, 2005), and wherein the star
formation appears to be progressing in stages over timescales that far exceed
the individual collapse times for massive star progenitors. This suggests a
stimulus that triggers the star formation, perhaps provided by a “daisy chain”
effect in which newly formed stars trigger collapse in nearby parts of the
cloud. Similar suggestions are proposed in the 30 Dor region surrounding R136
(Walborn, Maíz-Apellániz, & Barbá, 2002). While there is no evidence for an
age-dispersed population in Sgr B2, Sato et al. (2000) suggest that the cloud
was triggered to form stars through a cloud-cloud interaction.

##  9 Conclusions

Massive star formation in the GC has produced an extraordinary sample of stars
populating the initial mass function up to a cutoff of approximately 150 M⊙.
The ranges of inferred masses and observed spectral types are as expected from
stellar evolution models, and the extraordinary distribution of stars in the
region is a direct consequence of the large amount of mass that has fed star
formation in the GC. The origin of the massive stars in the central parsec,
and especially the central arcsecond, remains unresolved.

###### Acknowledgements.

I thank the following individuals for discussions related to this work: Mark
Morris, Bob Blum, Reinhard Genzel, Paco Najarro, Sungsoo Kim, and Peter
Tuthill. Tal Alexander made substantial contributions to Table 3. The material
in this paper is based upon work supported by NASA under award No.
NNG05-GC37G, through the Long Term Space Astrophysics program. Full resolution
versions of the above images are available at http://www.cis.rit.edu/
dffpci/private/papers/stsci06/

## References

  * Alexander (1999) Alexander, T. 1999 The Distribution of Stars near the Supermassive Black Hole in the Galactic Center. Astrophysical Journal 527, 835
  * Alexander (2005) Alexander, T. 2005, Stellar processes near the massive black hole in the Galactic center. Physics Reports 419, 65
  * Alexander & Livio (2004) Alexander, T., & Livio, M. 2004 Orbital Capture of Stars by a Massive Black Hole via Exchanges with Compact Remnants. Astrophysical Journal Letters 606, L21
  * Alexander & Morris (2003) Alexander, T., & Morris, M. 2003 Squeezars: Tidally Powered Stars Orbiting a Massive Black Hole. Astrophysical Journal Letters 590, L25
  * Allen, Hyland, & Hillier (1990) Allen, D. A., Hyland, A. R., & Hillier, D. J. 1990 The source of luminosity at the Galactic Centre. Monthly Notices of the Royal Astronomical Society 244, 706
  * Bailey & Davies (1999) Bailey, V. C., & Davies, M. B. 1999 Red giant collisions in the Galactic Centre. Monthly Notices of the Royal Astronomical Society 308, 257
  * Becklin et al. (1978) Becklin, E. E., Matthews, K., Neugebauer, G., & Willner, S. P. 1978 Infrared observations of the galactic center. I - Nature of the compact sources. Astrophysical Journal 219, 121
  * Beloborodov et al. (2006) Beloborodov, A. M., Levin, Y., Eisenhauer, F., Genzel, R., Paumard, T., Gillessen, S., & Ott, T. 2006 Clockwise Stellar Disk and the Dark Mass in the Galactic Center. ArXiv Astrophysics e-prints , arXiv:astro-ph/0601273
  * Berukoff & Hansen (2006) Berukoff, S. J., & Hansen, B. M. S. 2006 Cluster Core Dynamics at the Galactic Center. ArXiv Astrophysics e-prints , arXiv:astro-ph/0607080
  * Bica et al. (2003) Bica, E., Dutra, C. M., Soares, J., & Barbuy, B. 2003 New infrared star clusters in the Northern and Equatorial Milky Way with 2MASS. Astronomy & Astrophysics 404, 223
  * Blum (1995) Blum, R. D. 1995 The Stellar Population at the Galactic Center and the Mass Distribution in the Inner Galaxy. Ph.D. Thesis
  * Blum, Depoy, & Sellgren (1995a) Blum, R. D., Depoy, D. L., & Sellgren, K. 1995a A comparison of near-infrared spectra of the galactic center compact He I emission-line sources and early-type mass-losing stars. Astrophysical Journal 441, 603
  * Blum et al. (2003) Blum, R. D., Ramírez, S. V., Sellgren, K., & Olsen, K. 2003 Really Cool Stars and the Star Formation History at the Galactic Center. Astrophysical Journal 597, 323
  * Blum et al. (2001) Blum, R. D., Schaerer, D., Pasquali, A., Heydari-Malayeri, M., Conti, P. S., & Schmutz, W. 2001 2 Micron Narrowband Adaptive Optics Imaging in the Arches Cluster. Astronomical Journal 122, 1875
  * Blum, Sellgren, & Depoy (1995b) Blum, R. D., Sellgren, K., & Depoy, D. L. 1995b Discovery of a possible Wolf-Rayet star at the galactic center. Astrophysical Journal Letters 440, L17
  * Blum, Sellgren, & Depoy (1996a) Blum, R. D., Sellgren, K., & Depoy, D. L. 1996a Really Cool Stars at the Galactic Center. Astronomical Journal 112, 1988
  * Blum, Sellgren, & Depoy (1996b) Blum, R. D., Sellgren, K., & Depoy, D. L. 1996b JHKL Photometry and the K-Band Luminosity Function at the Galactic Center. Astrophysical Journal 470, 864
  * Carr, Sellgren, & Balachandran (2000) Carr, J. S., Sellgren, K., & Balachandran, S. C. 2000 The First Stellar Abundance Measurements in the Galactic Center: The M Supergiant IRS 7\. Astrophysical Journal 530, 307
  * Chiar et al. (2003) Chiar, J. E., Adamson, A. J., Whittet, D. C. B., & Pendleton, Y. J. 2003 Spectroscopy of Hydrocarbon Grains toward the Galactic Center and Quintuplet Cluster. Astronomische Nachrichten Supplement 324, 109
  * Chuss et al. (2003) Chuss, D. T., Davidson, J. A., Dotson, J. L., Dowell, C. D., Hildebrand, R. H., Novak, G., & Vaillancourt, J. E. 2003 Magnetic Fields in Cool Clouds within the Central 50 Parsecs of the Galaxy. Astrophysical Journal 599, 1116
  * Clark et al. (2005) Clark, J. S., Negueruela, I., Crowther, P. A., & Goodwin, S. P. 2005 On the massive stellar population of the super star cluster Westerlund 1. Astronomy & Astrophysics 434, 949
  * Cotera (1995) Cotera, A. S. 1995 Stellar Ionization of the Thermal Emission Regions in the Galactic Center. Ph.D. Thesis
  * Cotera et al. (1996) Cotera, A. S., Erickson, E. F., Colgan, S. W. J., Simpson, J. P., Allen, D. A., & Burton, M. G. 1996 The Discovery of Hot Stars near the Galactic Center Thermal Radio Filaments. Astrophysical Journal 461, 750
  * Cotera et al. (1999) Cotera, A. S., Simpson, J. P., Erickson, E. F., Colgan, S. W. J., Burton, M. G., & Allen, D. A. 1999 Isolated Hot Stars in the Galactic Center Vicinity. Astrophysical Journal 510, 747
  * Crowther et al. (2006) Crowther, P.A., Hadfield, L. J., Clark, J. S., Negueruela, I., & Vacca, W. D. 2006 A census of the Wolf-Rayet content in Westerlund 1 from near-infrared imaging and spectroscopy. ArXiv Astrophysics e-prints , arXiv:astro-ph/0608356
  * Davies et al. (1998) Davies, M. B., Blackwell, R., Bailey, V. C., & Sigurdsson, S. 1998 The destructive effects of binary encounters on red giants in the Galactic Centre. Monthly Notices of the Royal Astronomical Society 301, 745
  * Davies & King (2005) Davies, M. B., & King, A. 2005 The Stars of the Galactic Center. Astrophysical Journal Letters 624, L25
  * de Pree et al. (1995) de Pree, C. G., Gaume, R. A., Goss, W. M., & Claussen, M. J. 1995 The Sagittarius B2 Star-forming Region. II. High-Resolution H66 alpha Observations of Sagittarius B2 North. Astrophysical Journal 451, 284
  * de Pree et al. (1996) de Pree, C. G., Gaume, R. A., Goss, W. M., & Claussen, M. J. 1996 The Sagittarius B2 Star-forming Region. III. High-Resolution H52 alpha and H66 alpha Observations of Sagittarius B2 Main. Astrophysical Journal 464, 788
  * de Vicente et al. (2000) de Vicente, P., Martín-Pintado, J., Neri, R., & Colom, P. 2000 A ridge of recent massive star formation between Sgr B2M and Sgr B2N. Astronomy & Astrophysics 361, 1058
  * Dray, King, & Davies (2006) Dray, L. M., King, A. R., & Davies, M. B. 2006 Young stars in the Galactic Centre: a potential intermediate-mass star origin. ArXiv Astrophysics e-prints , arXiv:astro-ph/0607470
  * Dutra et al. (2003) Dutra, C. M., Bica, E., Soares, J., & Barbuy, B. 2003 New infrared star clusters in the southern Milky Way with 2MASS. Astronomy & Astrophysics 400, 533
  * Eckart et al. (2002) Eckart, A., Genzel, R., Ott, T., & Schödel, R. 2002 Stellar orbits near Sagittarius A*. Monthly Notices of the Royal Astronomical Society 331, 917
  * Eckart et al. (1999) Eckart, A., Ott, T., & Genzel, R. 1999, The Sgr A* stellar cluster: New NIR imaging and spectroscopy. Astronomy & Astrophysics 352, L22
  * Eckart et al. (2005) Eckart, A., Schödel, R., & Straubmeier, C. 2005 The black hole at the center of the Milky Way. Andreas Eckart, Rainer Schödel, Christian Straubmeier. London: Imperial College Press, ISBN 1-86094-567-8, 2005, XXII+284
  * Eikenberry et al. (2004) Eikenberry, S. S., et al. 2004 Infrared Observations of the Candidate LBV 1806-20 and Nearby Cluster Stars1,. Astrophysical Journal 616, 506
  * Eisenhauer et al. (2005) Eisenhauer, F., et al. 2005 SINFONI in the Galactic Center: Young Stars and Infrared Flares in the Central Light-Month. Astrophysical Journal 628, 246
  * Eisenhauer et al. (2003) Eisenhauer, F., Schödel, R., Genzel, R., Ott, T., Tecza, M., Abuter, R., Eckart, A., & Alexander, T. 2003 A Geometric Determination of the Distance to the Galactic Center. Astrophysical Journal Letters 597, L121
  * Figer (1995) Figer, D. F. 1995 A Search for Emission-Line Stars Near the Galactic Center. Ph.D. Thesis.
  * Figer (2003) Figer, D. F. 2003, Massive stars and the creation of our Galactic Center. IAU Symposium, 212, 487
  * Figer (2004) Figer, D. F. 2004, Young Massive Clusters in the Galactic Center. ASP Conf. Ser. 322: The Formation and Evolution of Massive Young Star Clusters, 322, 49.
  * Figer (2005) Figer, D. F. 2005 An upper limit to the masses of stars. Nature 434, 192
  * Figer et al. (2000) Figer, D. F., et al. 2000, 2 Micron Spectroscopy within 0.3” of Sagittarius A*. Astrophysical Journal Letters 533, L49
  * Figer et al. (2002) Figer, D. F. et al. 2002 Massive Stars in the Arches Cluster. Astrophysical Journal 581, 258
  * Figer et al. (2003) Figer, D. F., et al. 2003 High-Precision Stellar Radial Velocities in the Galactic Center. Astrophysical Journal 599, 1139
  * Figer et al. (1994) Figer, D. F., Becklin, E. E., McLean, I. S., & Morris, M. 1994 Discovery of Luminous NIR Sources Associated With Ionized Gas Near the Galactic Center. ASSL Vol. 190: Astronomy with Arrays, The Next Generation , 545
  * Figer & Kim (2002) Figer, D. F., & Kim, S. S. 2002 Stellar Collisions and Mergers in the Galactic Center. ASP Conf. Ser. 263: Stellar Collisions, Mergers and their Consequences 263, 287
  * Figer et al. (1999b) Figer, D. F., Kim, S. S., Morris, M., Serabyn, E., Rich, R. M., & McLean, I. S. 1999b HST/NICMOS Observations of Massive Stellar Clusters Near the Galactic Center. Astrophysical Journal 525, 750.
  * Figer et al. (2006) Figer, D. F., MacKenty, J. W., Robberto, M., Smith, K., Najarro, F., Kudritzki, R. P., & Herrero, A. 2006 Discovery of an Extraordinarily Massive Cluster of Red Supergiants. Astrophysical Journal 643, 1166
  * Figer, McLean, & Morris (1995) Figer, D. F., McLean, I. S., & Morris, M. 1995 Two New Wolf-Rayet Stars and a Luminous Blue Variable Star in the Quintuplet (AFGL 2004) near the Galactic Center. Astrophysical Journal Letters 447, L29
  * Figer et al. (1999a) Figer, D. F., McLean, I. S., & Morris, M. 1999a, Massive Stars in the Quintuplet Cluster. Astrophysical Journal 514, 202
  * Figer et al. (1999c) Figer, D. F., Morris, M., Geballe, T. R., Rich, R. M., Serabyn, E., McLean, I. S., Puetter, R. C., & Yahil, A. 1999c High-Resolution Infrared Imaging and Spectroscopy of the Pistol Nebula: Evidence for Ejection. Astrophysical Journal 525, 759
  * Figer, Morris, & McLean (1996) Figer, D. F., Morris, M., & McLean, I. S. 1996 Hot Stars in the Quintuplet. ASP Conf. Ser. 102: The Galactic Center 102, 263
  * Figer et al. (2005) Figer, D. F., Najarro, F., Geballe, T. R., Blum, R. D., & Kudritzki, R. P. 2005 Massive Stars in the SGR 1806-20 Cluster. Astrophysical Journal Letters 622, L49
  * Figer, Najarro, & Kudritzki (2004) Figer, D. F., Najarro, F., & Kudritzki, R. P. 2004 The Double-lined Spectrum of LBV 1806-20. Astrophysical Journal Letters 610, L109
  * Figer et al. (1998) Figer, D. F., Najarro, F., Morris, M., McLean, I. S., Geballe, T. R., Ghez, A. M., & Langer, N. 1998 The Pistol star. Astrophysical Journal 506, 384
  * Figer et al. (2004) Figer, D. F., Rich, R. M., Kim, S. S., Morris, M., & Serabyn, E. 2004 An Extended Star Formation History for the Galactic Center from Hubble Space Telescope NICMOS Observations. Astrophysical Journal 601, 319
  * Forrest et al. (1987) Forrest, W. J., Shure, M. A., Pipher, J. L., & Woodward, C. E. 1987 Brackett Alpha Images. AIP Conf. Proc. 155: The Galactic Center 155, 153
  * Freitag, Amaro-Seoane, & Kalogera (2006) Freitag, M., Amaro-Seoane, P., & Kalogera, V. 2006 Stellar remnants in galactic nuclei: mass segregation. ArXiv Astrophysics e-prints , arXiv:astro-ph/0603280
  * Freitag, Rasio, & Baumgardt (2006) Freitag, M., Rasio, F. A., & Baumgardt, H. 2006 Runaway collisions in young star clusters - I. Methods and tests. Monthly Notices of the Royal Astronomical Society 368, 121
  * Frogel, Tiede, & Kuchinski (1999) Frogel, J. A., Tiede, G. P., & Kuchinski, L. E. 1999 The Metallicity and Reddening of Stars in the Inner Galactic Bulge. Astronomical Journal 117, 2296
  * Garay & Lizano (1999) Garay, G., & Lizano, S. 1999 Massive Stars: Their Environment and Formation. Publications of the Astronomical Society of the Pacific 111, 1049
  * Gaume et al. (1995) Gaume, R. A., Claussen, M. J., de Pree, C. G., Goss, W. M., & Mehringer, D. M. 1995 The Sagittarius B2 Star-forming Region. I. Sensitive 1.3 Centimeter Continuum Observations. Astrophysical Journal 449, 663
  * Geballe et al. (1994) Geballe, T. R., Genzel, R., Krabbe, A., Krenz, T., & Lutz, D. 1994 Spectra of a Remarkable Class of Hot Stars in the Galactic Center. ASSL Vol. 190: Astronomy with Arrays, The Next Generation , 73
  * Geballe, Najarro, & Figer (2000) Geballe, T. R., Najarro, F., & Figer, D. F. 2000 A Second Luminous Blue Variable in the Quintuplet Cluster. Astrophysical Journal Letters 530, L97
  * Genzel et al. (2003) Genzel, R., et al. 2003 The Stellar Cusp around the Supermassive Black Hole in the Galactic Center. Astrophysical Journal 594, 812
  * Genzel et al. (1997) Genzel, R., Eckart, A., Ott, T., & Eisenhauer, F. 1997, On the nature of the dark mass in the centre of the Milky Way. Monthly Notices of the Royal Astronomical Society 291, 219
  * Genzel et al. (1994) Genzel, R., Hollenbach, D., & Townes, C. H. 1994, The nucleus of our Galaxy. Reports of Progress in Physics, 57, 417.
  * Genzel et al. (1996) Genzel, R., Thatte, N., Krabbe, A., Kroker, H., & Tacconi-Garman, L. E. 1996 The Dark Mass Concentration in the Central Parsec of the Milky Way. Astrophysical Journal 472, 153
  * Genzel & Townes (1987) Genzel, R., & Townes, C. H. 1987 Physical conditions, dynamics, and mass distribution in the center of the Galaxy. Annual Review of Astronomy and Astrophysics 25, 377
  * Gerhard (2001) Gerhard, O. 2001 The Galactic Center HE I Stars: Remains of a Dissolved Young Cluster?. Astrophysical Journal Letters 546, L39
  * Ghez et al. (2003) Ghez, A. M., et al. 2003, The First Measurement of Spectral Lines in a Short-Period Star Bound to the Galaxy’s Central Black Hole: A Paradox of Youth. Astrophysical Journal Letters 586, L127
  * Ghez et al. (1998) Ghez, A. M., Klein, B. L., Morris, M., & Becklin, E. E. 1998 High Proper-Motion Stars in the Vicinity of Sagittarius A*: Evidence for a Supermassive Black Hole at the Center of Our Galaxy. Astrophysical Journal 509, 678
  * Ghez et al. (2000) Ghez, A. M., Morris, M., Becklin, E. E., Tanner, A., & Kremenek, T. 2000 The accelerations of stars orbiting the Milky Way’s central black hole. Nature 407, 349
  * Ghez et al. (2005) Ghez, A. M., Salim, S., Hornstein, S. D., Tanner, A., Lu, J. R., Morris, M., Becklin, E. E., & Duchêne, G. 2005 Stellar Orbits around the Galactic Center Black Hole. Astrophysical Journal 620, 744
  * Glass et al. (2001) Glass, I. S., Matsumoto, S., Carter, B. S., & Sekiguchi, K. 2001 Large-amplitude variables near the Galactic Centre. Monthly Notices of the Royal Astronomical Society 321, 77
  * Glass, Moneti, & Moorwood (1990) Glass, I. S., Moneti, A., & Moorwood, A. F. M. 1990 Infrared images and photometry of the cluster near G 0.15 - 0.05. Monthly Notices of the Royal Astronomical Society 242, 55P
  * Goodman & Paczynski (2005) Goodman, J., & Paczynski, B. 2005 On the nature of the S stars in the Galactic Center. ArXiv Astrophysics e-prints, arXiv:astro-ph/0504079
  * Goss et al. (1985) Goss, W. M., Schwarz, U. J., van Gorkom, J. H., & Ekers, R. D. 1985 The SGR A East H II complex at L = -0.02, B = -0.07 deg. Monthly Notices of the Royal Astronomical Society 215, 69P
  * Gould & Quillen (2003) Gould, A., & Quillen, A. C. 2003 Sagittarius A* Companion S0-2: A Probe of Very High Mass Star Formation. Astrophysical Journal 592, 935
  * Groh et al. (2006) Groh, J. H., Damineli, A., Teodoro, M., & Barbosa, C. L. 2006 Detection of additional Wolf-Rayet stars in the starburst cluster Westerlund 1 with SOAR. ArXiv Astrophysics e-prints , arXiv:astro-ph/0606498
  * Gürkan & Rasio (2005) Gürkan, M. A., & Rasio, F. A. 2005 The Disruption of Stellar Clusters Containing Massive Black Holes near the Galactic Center. Astrophysical Journal 628, 236
  * Haislip & Youdin (2005) Haislip, G., & Youdin, A. N. 2005 A Disk Origin for S-Stars in the Galactic Center? American Astronomical Society Meeting Abstracts 207, #181.14
  * Haller & Melia (1996) Haller, J. W., & Melia, F. 1996 Inferring Spherical Mass Distributions Using the Projected Mass Estimator. Astrophysical Journal 464, 774
  * Haller et al. (1996) Haller, J. W., Rieke, M. J., Rieke, G. H., Tamblyn, P., Close, L., & Melia, F. 1996 Stellar Kinematics and the Black Hole in the Galactic Center. Astrophysical Journal 456, 194
  * Hansen & Milosavljević (2003) Hansen, B. M. S., & Milosavljević, M. 2003 The Need for a Second Black Hole at the Galactic Center. Astrophysical Journal Letters 593, L77
  * Harris et al. (1994) Harris, A. I., Krenz, T., Genzel, R., Krabbe, A., Lutz, D., Politsch, A., Townes, C. H., & Geballe, T. R. 1994 Spectroscopy of the Galactic Center Arches Region: Evidence for Massive Star Formation. NATO ASIC Proc. 445: The Nuclei of Normal Galaxies: Lessons from the Galactic Center , 223
  * Hildebrand et al. (1993) Hildebrand, R. H., Davidson, J. A., Dotson, J., Figer, D. F., Novak, G., Platt, S. R., & Tao, L. 1993 Polarization of the Thermal Emission from the Dust Ring at the Center of the Galaxy. Astrophysical Journal 417, 565
  * Homeier & Alves (2005) Homeier, N. L., & Alves, J. 2005 Massive star formation in the W49 giant molecular cloud: Implications for the formation of massive star clusters. Astronomy & Astrophysics 430, 481
  * Homeier et al. (2003) Homeier, N. L., Blum, R. D., Pasquali, A., Conti, P. S., & Damineli, A. 2003 Results from a near infrared search for emission-line stars in the Inner Galaxy: Spectra of new Wolf-Rayet stars. Astronomy & Astrophysics 408, 153
  * Hopman & Alexander (2006) Hopman, C., & Alexander, T. 2006 Resonant Relaxation near the Massive Black Hole in the Galactic Center. ArXiv Astrophysics e-prints , arXiv:astro-ph/0605457
  * Humphreys & Davidson (1994) Humphreys, R. M., & Davidson, K. 1994 The luminous blue variables: Astrophysical geysers. Publications of the Astronomical Society of the Pacific 106, 1025
  * Kennicutt (1998) Kennicutt, R. C., Jr. 1998 Star Formation in Galaxies Along the Hubble Sequence. Annual Review of Astronomy and Astrophysics 36, 189
  * Kim et al. (2000) Kim, S. S., Figer, D. F., Lee, H. M., & Morris, M. 2000 N-Body Simulations of Compact Young Clusters near the Galactic Center. Astrophysical Journal 545, 301
  * Kim, Figer, & Morris (2004) Kim, S. S., Figer, D. F., & Morris, M. 2004 Dynamical Friction on Galactic Center Star Clusters with an Intermediate-Mass Black Hole. Astrophysical Journal Letters 607, L123
  * Kim et al. (2006) Kim, S. S., Figer, D. F., Kudritzki, R. P., & Najarro, F. N. 2006 Keck/LGS Observations of the Arches Cluster. Astrophysical Journal submitted
  * Kim & Morris (2003) Kim, S. S., & Morris, M. 2003 Dynamical Friction on Star Clusters near the Galactic Center. Astrophysical Journal 597, 312
  * Koen (2006) Koen, C. 2006 On the upper limit on stellar masses in the Large Magellanic Cloud cluster R136. Monthly Notices of the Royal Astronomical Society 365, 590
  * Krabbe et al. (1995) Krabbe, A., et al. 1995 The Nuclear Cluster of the Milky Way: Star Formation and Velocity Dispersion in the Central 0.5 Parsec. Astrophysical Journal Letters 447, L95
  * Krabbe et al. (1991) Krabbe, A., Genzel, R., Drapatz, S., & Rotaciuc, V. 1991 A cluster of He I emission-line stars in the Galactic center. Astrophysical Journal Letters 382, L19
  * Lacy, Achtermann, & Serabyn (1991) Lacy, J. H., Achtermann, J. M., & Serabyn, E. 1991 Galactic center gasdynamics - A one-armed spiral in a Keplerian disk. Astrophysical Journal Letters 380, L71
  * Lacy et al. (1980) Lacy, J. H., Townes, C. H., Geballe, T. R., & Hollenbach, D. J. 1980 Observations of the motion and distribution of the ionized gas in the central parsec of the Galaxy. II. Astrophysical Journal 241, 132
  * Lacy, Townes, & Hollenbach (1982) Lacy, J. H., Townes, C. H., & Hollenbach, D. J. 1982 The nature of the central parsec of the Galaxy. Astrophysical Journal 262, 120
  * Lang et al. (1999) Lang, C. C., Figer, D. F., Goss, W. M., & Morris, M. 1999 Radio Detections of Stellar Winds from the Pistol star and Other Stars in the Galactic Center Quintuplet Cluster. Astronomical Journal 118, 2327
  * Lang, Goss, & Rodríguez (2001) Lang, C. C., Goss, W. M., & Rodríguez, L. F. 2001 Very Large Array Detection of the Ionized Stellar Winds Arising from Massive Stars in the Galactic Center Arches Cluster. Astrophysical Journal Letters 551, L143
  * Lang, Goss, & Wood (1997) Lang, C. C., Goss, W. M., & Wood, D. O. S. 1997 VLA H92 alpha and H115 beta Recombination Line Observations of the Galactic Center H II Regions: The Sickle (G0.18-0.04) and the Pistol (G0.15-0.05). Astrophysical Journal 474, 275
  * Lang et al. (2005) Lang, C. C., Johnson, K. E., Goss, W. M., & Rodríguez, L. F. 2005 Stellar Winds and Embedded Star Formation in the Galactic Center Quintuplet and Arches Clusters: Multifrequency Radio Observations. Astronomical Journal 130, 2185
  * Law & Yusef-Zadeh (2004) Law, C., & Yusef-Zadeh, F. 2004 X-Ray Observations of Stellar Clusters Near the Galactic Center. Astrophysical Journal 611, 858
  * Lebofsky & Rieke (1987) Lebofsky, M. J., & Rieke, G. H. 1987 The Stellar Population. AIP Conf. Proc. 155: The Galactic Center 155, 79
  * Lebofsky, Rieke, & Tokunaga (1982) Lebofsky, M. J., Rieke, G. H., & Tokunaga, A. T. 1982 M supergiants and star formation at the galactic center. Astrophysical Journal 263, 736
  * Levin (2006) Levin, Y. 2006 Starbursts near supermassive black holes: young stars in the Galactic Center, and gravitational waves in LISA band. ArXiv Astrophysics e-prints , arXiv:astro-ph/0603583
  * Levin & Beloborodov (2003) Levin, Y., & Beloborodov, A. M. 2003 Stellar Disk in the Galactic Center: A Remnant of a Dense Accretion Disk? Astrophysical Journal Letters 590, L33
  * Levin, Wu, & Thommes (2005) Levin, Y., Wu, A., & Thommes, E. 2005 Intermediate-Mass Black Hole(s) and Stellar Orbits in the Galactic Center. Astrophysical Journal 635, 341
  * Libonate et al. (1995) Libonate, S., Pipher, J. L., Forrest, W. J., & Ashby, M. L. N. 1995 Near-infrared spectra of compact stellar wind sources at the Galactic center. Astrophysical Journal 439, 202
  * Lindqvist, Habing, & Winnberg (1992) Lindqvist, M., Habing, H. J., & Winnberg, A. 1992 OH/IR stars close to the Galactic Centre. II - Their spatial and kinematic properties and the mass distribution within 5-100 PC from the galactic centre. Astronomy & Astrophysics 259, 118
  * Liu & Snyder (1999) Liu, S.-Y., & Snyder, L. E. 1999 Subarcsecond Resolution Observations of Sagittarius B2 at 85 GHZ. Astrophysical Journal 523, 683
  * Lynden-Bell & Rees (1971) Lynden-Bell, D., & Rees, M. J. 1971 On quasars, dust and the galactic centre. Monthly Notices of the Royal Astronomical Society 152, 461
  * Maeda et al. (2002) Maeda, Y., et al. 2002 A Chandra Study of Sagittarius A East: A Supernova Remnant Regulating the Activity of Our Galactic Center?. Astrophysical Journal 570, 671
  * Mason et al. (1998) Mason, B. D., Gies, D. R., Hartkopf, W. I., Bagnuolo, W. G., Jr., ten Brummelaar, T., & McAlister, H. A. 1998 ICCD speckle observations of binary stars. XIX - an astrometric/spectroscopic survey of O stars. Astronomical Journal 115, 821
  * Massey & Hunter (1998) Massey, P., & Hunter, D. A. 1998 Star Formation in R136: A Cluster of O3 Stars Revealed by Hubble Space Telescope Spectroscopy. Astrophysical Journal 493, 180
  * McGinn et al. (1989) McGinn, M. T., Sellgren, K., Becklin, E. E., & Hall, D. N. B. 1989 Stellar kinematics in the Galactic center. Astrophysical Journal 338, 824
  * McGrath, Goss, & De Pree (2004) McGrath, E. J., Goss, W. M., & De Pree, C. G. 2004 H2O Masers in W49 North and Sagittarius B2. Astrophysical Journal Supplements 155, 577
  * McMillan & Portegies Zwart (2003) McMillan, S. L. W., & Portegies Zwart, S. F. 2003 The Fate of Star Clusters near the Galactic Center. I. Analytic Considerations. Astrophysical Journal 596, 314
  * Mercer et al. (2005) Mercer, E. P., et al. 2005 New Star Clusters Discovered in the GLIMPSE Survey. Astrophysical Journal 635, 560
  * Milosavljević & Loeb (2004) Milosavljević, M., & Loeb, A. 2004 The Link between Warm Molecular Disks in Maser Nuclei and Star Formation near the Black Hole at the Galactic Center. Astrophysical Journal Letters 604, L45
  * Moffat et al. (2002) Moffat, A. F. J., et al. 2002 Galactic Starburst NGC 3603 from X-Rays to Radio. Astrophysical Journal 573, 191
  * Moneti, Glass, & Moorwood (1994) Moneti, A., Glass, I. S., & Moorwood, A. F. M. 1994 Spectroscopy and Further Imaging of IRAS Sources Near the Galactic Centre. Monthly Notices of the Royal Astronomical Society 268, 194
  * Moneti et al. (2001) Moneti, A., Stolovy, S., Blommaert, J. A. D. L., Figer, D. F., & Najarro, F. 2001 Mid-infrared imaging and spectroscopy of the enigmatic cocoon stars in the Quintuplet Cluster. Astronomy & Astrophysics 366, 106
  * Monnier, Tuthill, & Danchi (1999) Monnier, J. D., Tuthill, P. G., & Danchi, W. C. 1999 Pinwheel Nebula around WR 98A. Astrophysical Journal Letters 525, L97
  * Morris (1993) Morris, M. 1993 Massive star formation near the Galactic center and the fate of the stellar remnants. Astrophysical Journal 408, 496
  * Morris, Ghez, & Becklin (1999) Morris, M., Ghez, A. M., & Becklin, E. E. 1999 The galactic center black hole: clues for the evolution of black holes in galactic nuclei. Advances in Space Research 23, 959
  * Morris & Serabyn (1996) Morris, M., & Serabyn, E. 1996 The Galactic Center Environment. Annual Review of Astronomy and Astrophysics 34, 645
  * Muno et al. (2006) Muno, M. P., Bower, G. C., Burgasser, A. J., Baganoff, F. K., Morris, M. R., & Brandt, W. N. 2006 Isolated, Massive Supergiants near the Galactic Center. Astrophysical Journal 638, 183
  * Nagata et al. (1995) Nagata, T., Woodward, C. E., Shure, M., & Kobayashi, N. 1995 Object 17: Another cluster of emission-line stars near the galactic center. Astronomical Journal 109, 1676
  * Nagata et al. (1990) Nagata, T., Woodward, C. E., Shure, M., Pipher, J. L., & Okuda, H. 1990 AFGL 2004 - an infrared quintuplet near the Galactic center. Astrophysical Journal 351, 83
  * Najarro (1995) Najarro, F. 1995 Quantitative Optical and Infrared Spectroscopy of Extreme Luminous Blue Supergiants. Ph.D. Thesis ,
  * Najarro et al. (2004) Najarro, F., Figer, D. F., Hillier, D. J., & Kudritzki, R. P. 2004 Metallicity in the Galactic Center: The Arches Cluster. Astrophysical Journal Letters 611, L105
  * Najarro et al. (1994) Najarro, F., Hillier, D. J., Kudritzki, R. P., Krabbe, A., Genzel, R., Lutz, D., Drapatz, S., & Geballe, T. R. 1994 The nature of the brightest galactic center HeI emission line star. Astronomy & Astrophysics 285, 573
  * Najarro et al. (1997) Najarro, F., Krabbe, A., Genzel, R., Lutz, D., Kudritzki, R. P., & Hillier, D. J. 1997 Quantitative spectroscopy of the HeI cluster in the Galactic center.. Astronomy & Astrophysics 325, 700
  * Nayakshin & Cuadra (2005) Nayakshin, S., & Cuadra, J. 2005 A self-gravitating accretion disk in Sgr A* a few million years ago: Is Sgr A* a failed quasar?. Astronomy & Astrophysics 437, 437
  * Narayanan, Gould, & Depoy (1996) Narayanan, V. K., Gould, A., & Depoy, D. L. 1996 Luminosity Function of the Perigalactocentric Region. Astrophysical Journal 472, 183
  * Nayakshin & Sunyaev (2005) Nayakshin, S., & Sunyaev, R. 2005 The ‘missing’ young stellar objects in the central parsec of the Galaxy: evidence for star formation in a massive accretion disc and a top-heavy initial mass function. Monthly Notices of the Royal Astronomical Society 364, L23
  * Negueruela & Clark (2005) Negueruela, I., & Clark, J. S. 2005 Further Wolf-Rayet stars in the starburst cluster Westerlund 1. Astronomy & Astrophysics 436, 541
  * Nelan et al. (2004) Nelan, E. P., Walborn, N. R., Wallace, D. J., Moffat, A. F. J., Makidon, R. B., Gies, D. R., & Panagia, N. 2004 Resolving OB Systems in the Carina Nebula with the Hubble Space Telescope Fine Guidance Sensor. Astronomical Journal 128, 323
  * Oey & Clarke (2005) Oey, M. S., & Clarke, C. J. 2005 Statistical Confirmation of a Stellar Upper Mass Limit. Astrophysical Journal Letters 620, L43
  * Okuda et al. (1990) Okuda, H., et al. 1990 An infrared quintuplet near the Galactic center. Astrophysical Journal 351, 89
  * Paumard et al. (2006) Paumard, T., et al. 2006, The Two Young Star Disks in the Central Parsec of the Galaxy: Properties, Dynamics, and Formation. Astrophysical Journal 643, 1011
  * Perets, Hopman, & Alexander (2006) Perets, H. B., Hopman, C., & Alexander, T. 2006 Massive perturber-driven interactions of stars with a massive black hole. ArXiv Astrophysics e-prints , arXiv:astro-ph/0606443
  * Phinney (1989) Phinney, E. S. 1989 Manifestations of a Massive Black Hole in the Galactic Center. IAU Symp. 136: The Center of the Galaxy 136, 543
  * Plante, Lo, & Crutcher (1995) Plante, R. L., Lo, K. Y., & Crutcher, R. M. 1995 The magnetic fields in the galactic center: Detection of H1 Zeeman splitting. Astrophysical Journal Letters 445, L113
  * Portegies Zwart et al. (2002) Portegies Zwart, S. F., Makino, J., McMillan, S. L. W., & Hut, P. 2002 The Lives and Deaths of Star Clusters near the Galactic Center. Astrophysical Journal 565, 265
  * Portegies Zwart, McMillan, & Gerhard (2003) Portegies Zwart, S. F., McMillan, S. L. W., & Gerhard, O. 2003 The Origin of IRS 16: Dynamically Driven In-Spiral of a Dense Star Cluster to the Galactic Center? Astrophysical Journal 593, 352
  * Ramírez et al. (2000) Ramírez, S. V., Sellgren, K., Carr, J. S., Balachandran, S. C., Blum, R., Terndrup, D. M., & Steed, A. 2000 Stellar Iron Abundances at the Galactic Center. Astrophysical Journal 537, 205
  * Rieke & Rieke (1988) Rieke, G. H., & Rieke, M. J. 1988 Stellar velocities and the mass distribution in the Galactic center. Astrophysical Journal Letters 330, L33
  * Rieke, Telesco, & Harper (1978) Rieke, G. H., Telesco, C. M., & Harper, D. A. 1978 The infrared emission of the Galactic center. Astrophysical Journal 220, 556
  * Rockefeller et al. (2005) Rockefeller, G., Fryer, C. L., Melia, F., & Wang, Q. D. 2005 Diffuse X-Rays from the Arches and Quintuplet Clusters. Astrophysical Journal 623, 171
  * Salpeter (1955) Salpeter, E. E. 1955 The Luminosity Function and Stellar Evolution. Astrophysical Journal 121, 161
  * Sato et al. (2000) Sato, F., Hasegawa, T., Whiteoak, J. B., & Miyawaki, R. 2000 Cloud Collision-induced Star Formation in Sagittarius B2. I. Large-Scale Kinematics. Astrophysical Journal 535, 857
  * Schödel et al. (2002) Schödel, R., et al. 2002 A star in a 15.2-year orbit around the supermassive black hole at the centre of the Milky Way. Nature 419, 694
  * Schödel et al. (2003) Schödel, R., Ott, T., Genzel, R., Eckart, A., Mouawad, N., & Alexander, T. 2003 Stellar Dynamics in the Central Arcsecond of Our Galaxy. Astrophysical Journal 596, 1015
  * Schödel et al. (2006) Schödel, R., et al. 2006 Astronomy & Astrophysics , in preparation
  * Sellgren et al. (1987) Sellgren, K., Hall, D. N. B., Kleinmann, S. G., & Scoville, N. Z. 1987 Radial velocities of late-type stars in the galactic center. Astrophysical Journal 317, 881
  * Sellgren et al. (1990) Sellgren, K., McGinn, M. T., Becklin, E. E., & Hall, D. N. 1990 Velocity dispersion and the stellar population in the central 1.2 parsecs of the Galaxy. Astrophysical Journal 359, 112
  * Serabyn & Lacy (1985) Serabyn, E., & Lacy, J. H. 1985 Forbidden NE II observations of the galactic center - Evidence for a massive block hole. Astrophysical Journal 293, 445
  * Serabyn & Morris (1996) Serabyn, E., & Morris, M. 1996 Sustained star formation in the central stellar cluster of the Milky Way.. Nature 382, 602
  * Serabyn, Shupe, & Figer (1998) Serabyn, E., Shupe, D., & Figer, D. F. 1998 An extraordinary cluster of massive stars near the centre of the Milky Way.. Nature 394, 448
  * Shields & Ferland (1994) Shields, J. C., & Ferland, G. J. 1994 Nebular properties and the ionizing radiation field in the galactic center. Astrophysical Journal 430, 236
  * Sjouwerman et al. (1999) Sjouwerman, L. O., Habing, H. J., Lindqvist, M., van Langevelde, H. J., & Winnberg, A. 1999 OH/IR Stars as Signposts for Ancient Starburst Activity in the Galactic Center. ASP Conf. Ser. 186: The Central Parsecs of the Galaxy 186, 379
  * Skinner et al. (2006) Skinner, S. L., Simmons, A. E., Zhekov, S. A., Teodoro, M., Damineli, A., & Palla, F. 2006 A Rich Population of X-Ray-emitting Wolf-Rayet Stars in the Galactic Starburst Cluster Westerlund 1. Astrophysical Journal Letters 639, L35
  * Smith (2006) Smith, N. 2006 Eruptive Mass Loss in Very Massive Stars and Population III Stars. ArXiv Astrophysics e-prints , arXiv:astro-ph/0607457
  * Stark et al. (1989) Stark, A. A., Bally, J., Wilson, R. W., & Pound, M. W. 1989 Molecular Line Observations of the Galactic Center Region. IAU Symp. 136: The Center of the Galaxy 136, 129
  * Stolte (2003) Stolte, A. 2003, PhD Thesis, University of Heidelberg
  * Stolte et al. (2003) Stolte, A., Brandner, W., Grebel, E. K., Figer, D. F., Eisenhauer, F., Lenzen, R., & Harayama, Y. 2003 NAOS-CONICA performance in a crowded field - the Arches cluster. The Messenger 111, 9
  * Stolte et al. (2005) Stolte, A., Brandner, W., Grebel, E. K., Lenzen, R., & Lagrange, A.-M. 2005 The Arches Cluster: Evidence for a Truncated Mass Function? Astrophysical Journal Letters 628, L113
  * Stolte et al. (2002) Stolte, A., Grebel, E. K., Brandner, W., & Figer, D. F. 2002 The mass function of the Arches cluster from Gemini adaptive optics data. Astronomy & Astrophysics 394, 459
  * Subr & Karas (2005) Subr, L. & Karas 2005 On highly eccentric stellar trajectories interacting with a self-gravitating disc in Sgr A*. Astronomy & Astrophysics 433, 405
  * Takagi, Murakami, & Koyama (2002) Takagi, S.-i., Murakami, H., & Koyama, K. 2002 X-Ray Sources and Star Formation Activity in the Sagittarius B2 Cloud Observed with Chandra. Astrophysical Journal 573, 275
  * Tamblyn et al. (1996) Tamblyn, P., Rieke, G. H., Hanson, M. M., Close, L. M., McCarthy, D. W., Jr., & Rieke, M. J. 1996 The Peculiar Population of Hot Stars at the Galactic Center. Astrophysical Journal 456, 206
  * Tanner et al. (2006) Tanner, A., et al. 2006 High Spectral Resolution Observations of the Massive Stars in the Galactic Center. Astrophysical Journal 641, 891
  * Timmermann et al. (1996) Timmermann, R., Genzel, R., Poglitsch, A., Lutz, D., Madden, S. C., Nikola, T., Geis, N., & Townes, C. H. 1996 Far-Infrared Observations of the Radio Arc (Thermal Arches) in the Galactic Center. Astrophysical Journal 466, 242
  * Tuthill, Monnier, & Danchi (1999) Tuthill, P. G., Monnier, J. D., & Danchi, W. C. 1999 A dusty pinwheel nebula around the massive star WR 104.. Nature 398, 487
  * Tuthill et al. (2006) Tuthill, P., Monnier, J., Tanner, A., Figer, D., & Ghez, A. 2006 “Pinwheels” discovered in the Quintuplet cluster. _Science_ 313, 935.
  * Uchida & Guesten (1995) Uchida, K. I., & Guesten, R. 1995 The large-scale magnetic field in the Galactic Center.. Astronomy & Astrophysics 298, 473
  * van der Hucht (2001) van der Hucht, K. A. 2001 The VIIth catalogue of galactic Wolf-Rayet stars. New Astronomy Review 45, 135
  * Walborn, Maíz-Apellániz, & Barbá (2002) Walborn, N. R., Maíz-Apellániz, J., & Barbá, R. H. 2002 Further Insights into the Structure of 30 Doradus from the Hubble Space Telescope Instruments. Astronomical Journal 124, 1601
  * Wang, Dong, & Lang (2006) Wang, Q. D., Dong, H., & Lang, C. 2006 The Interplay between Star Formation and the Nuclear Environment of our Galaxy: Deep X-ray Observations of the Galactic Center Arches and Quintuplet Clusters. ArXiv Astrophysics e-prints , arXiv:astro-ph/0606282
  * Weidner & Kroupa (2004) Weidner, C., & Kroupa, P. 2004 Evidence for a fundamental stellar upper mass limit from clustered star formation. Monthly Notices of the Royal Astronomical Society 348, 187
  * Weidner & Kroupa (2006) Weidner, C., & Kroupa, P. 2006 The maximum stellar mass, star-cluster formation and composite stellar populations. Monthly Notices of the Royal Astronomical Society 365, 1333
  * Williams, van der Hucht, & The (1987) Williams, P. M., van der Hucht, K. A., & The, P. S. 1987 Variable dust emission from Wolf-Rayet stars. Quarterly Journal of the Royal Astronomical Society 28, 248
  * Yusef-Zadeh et al. (2002) Yusef-Zadeh, F., Law, C., Wardle, M., Wang, Q. D., Fruscione, A., Lang, C. C., & Cotera, A. 2002 Detection of X-Ray Emission from the Arches Cluster near the Galactic Center. Astrophysical Journal 570, 665
  * Yusef-Zadeh & Morris (1987) Yusef-Zadeh, F., & Morris, M. 1987 Structural details of the Sagittarius A complex - Evidence for a large-scale poloidal magnetic field in the Galactic center region. Astrophysical Journal 320, 545
  * Yusef-Zadeh et al. (2003) Yusef-Zadeh, F., Nord, M., Wardle, M., Law, C., Lang, C., & Lazio, T. J. W. 2003 Nonthermal Emission from the Arches Cluster (G0.121+0.017) and the Origin of gamma-ray Emission from 3EG J1746-2851. Astrophysical Journal Letters 590, L103
  * Zhao et al. (1993) Zhao, J.-H., Desai, K., Goss, W. M., & Yusef-Zadeh, F. 1993 VLA Radio Recombination Line Observations of Ionized Gas in the -30 Kilometers per Second Molecular Cloud (G0.04+0.03) near the Galactic Center. I. The Discrete Radio Sources. Astrophysical Journal 418, 235