102 research outputs found
Extinction toward the Compact HII Regions G-0.02-0.07
The four HII regions in the Sgr A East complex: A, B, C, and D, represent
evidence of recent massive star formation in the central ten parsecs. Using
Paschen-alpha images taken with HST and 8.4 GHz VLA data, we construct an
extinction map of A-D, and briefly discuss their morphology and location.Comment: 2 pages, 1 figure. To be published in the Astronomical Society of the
Pacific Conference Series Proceedings of the Galactic Center Workshop 2009,
Shangha
HST Palpha Survey of the Galactic Center -- Searching the missing young stellar populations within the Galactic Center
We present preliminary results of our \hst Pa survey of the Galactic
Center (\gc), which maps the central 0.650.25 degrees around Sgr A*.
This survey provides us with a more complete inventory of massive stars within
the \gc, compared to previous observations. We find 157 Pa emitting
sources, which are evolved massive stars. Half of them are located outside of
three young massive star clusters near Sgr A*. The loosely spatial distribution
of these field sources suggests that they are within less massive star
clusters/groups, compared to the three massive ones. Our Pa mosaic not
only resolves previously well-known large-scale filaments into fine structures,
but also reveals many new extended objects, such as bow shocks and H II
regions. In particular, we find two regions with large-scale Pa diffuse
emission and tens of Pa emitting sources in the negative Galactic
longitude suggesting recent star formation activities, which were not known
previously. Furthermore, in our survey, we detect 0.6 million stars, most
of which are red giants or AGB stars. Comparisons of the magnitude distribution
in 1.90 m and those from the stellar evolutionary tracks with different
star formation histories suggest an episode of star formation process about 350
Myr ago in the \gc .Comment: 10 pages, 6 figures, Proceedings of the Galactic Center Workshop
2009, Shangha
The mid-infrared colors of the interstellar medium and extended sources at the Galactic center
A mid-infrared (3.6–8 μm) survey of the Galactic center has been carried out with the IRAC instrument on the Spitzer Space Telescope. This survey covers the central 2º x 1.4º (~280 x 200 pc) of the Galaxy. At 3.6 and 4.5 μm the emission is dominated by stellar sources, the fainter ones merging into an unresolved background. At 5.8 and 8 μm the stellar sources are fainter, and large-scale diffuse emission from the ISM of the Galaxy's central molecular zone becomes prominent. The survey reveals that the 8-to-5.8 μm color of the ISM emission is highly uniform across the surveyed region. This uniform color is consistent with a flat extinction law and emission from polycyclic aromatic hydrocarbons (PAHs). Models indicate that this broadband color should not be expected to change if the incident radiation field heating the dust and PAHs is ~10^4 times that of the solar neighborhood. Other regions of very red emission indicate cases where thick dust clouds obscure deeply embedded objects or very early stages of star formation
A mid-infrared survey of the inner 2 × 1.5 degrees of the Galaxy with Spitzer/IRAC
We present a survey of Spitzer Space Telescope/IRAC observations of the central 2 × 1.5 degrees (265 × 200 pc) of the Galaxy at 3-8 μm. These data represent the highest spatial resolution and sensitivity large-scale map made to date of the Galactic Center (GC) at mid-infrared wavelengths. The IRAC data provide a census of the optically obscured stellar sources as well as a detailed map of the highly filamentary structure in the interstellar medium. The diffuse emission is dominated by PAH emission from small grains in star-forming regions. Dark clouds displaying a large variety of sizes and morphologies are imaged, many of which remain opaque at IRAC wavelengths. Using a multiwavelength comparison, we determine which objects are likely to be in the foreground and which are located at the GC. We find no counterparts at IRAC wavelengths to the unique system of linear, nonthermal radio filaments present at the GC
Properties of the Compact HII Region Complex G-0.02-0.07
We present new extinction maps and high-resolution Paschen alpha images of
G-0.02-0.07, a complex of compact HII regions located adjacent to the
M-0.02-0.07 giant molecular cloud, 6 parsecs in projection from the center of
the Galaxy. These HII regions, which lie in projection just outside the
boundary of the Sgr A East supernova remnant, represent one of the most recent
episodes of star formation in the central parsecs of the Galaxy. The 1.87
micron extinctions of regions A, B and C are almost identical, approximately
1.5 magnitudes. Region D, in contrast, has a peak 1.87 micron extinction of 2.3
magnitudes. Adopting the Nishiyama et al. (2008) extinction law, we find these
extinctions correspond to visual extinctions of A_V = 44.5 and A_V = 70,
respectively. The similar and uniform extinctions of regions A, B and C are
consistent with that expected for foreground extinction in the direction of the
Galactic center, suggesting that they lie at the front side of the M-0.02-0.07
molecular cloud. Region D is more compact, has a higher extinction and is thus
suspected to be younger and embedded in a dense core in a compressed ridge on
the western edge of this cloud.Comment: 21 pages, 11 figures, submitted to ApJ, comments welcom
The Stellar Content of Obscured Galactic Giant H II Regions
Near infrared images of the Galactic giant HII region W43 reveal a dense
stellar cluster at its center. Broad band JHK photometry of the young cluster
and K-band spectra of three of its bright stars are presented. The 2 micron
spectrum of the brightest star in the cluster is very well matched to the
spectra of Wolf-Rayet stars of sub-type WN7. Two other stars are identified as
O type giants or supergiants by their NIII and CIV emission. The close spatial
clustering of O and the hydrogen WN type stars is analogous to the intense star
burst clusters R136 in the Large Magellanic Cloud and NGC3603 in the Galaxy.Comment: 22 pages (LaTex), including 7 figures (eps
HST/NICMOS Paschen-alpha Survey of the Galactic Center: Overview
We have recently carried out the first wide-field hydrogen Paschen-alpha line
imaging survey of the Galactic Center (GC), using the NICMOS instrument aboard
the Hubble Space Telescope. The survey maps out a region of 2253 pc^2 around
the central supermassive black hole (Sgr A*) in the 1.87 and 1.90 Micron narrow
bands with a spatial resolution of 0.01 pc at a distance of 8 kpc. Here we
present an overview of the observations, data reduction, preliminary results,
and potential scientific implications, as well as a description of the
rationale and design of the survey. We have produced mosaic maps of the
Paschen-alpha line and continuum emission, giving an unprecedentedly high
resolution and high sensitivity panoramic view of stars and photo-ionized gas
in the nuclear environment of the Galaxy. We detect a significant number of
previously undetected stars with Paschen-alpha in emission. They are most
likely massive stars with strong winds, as confirmed by our initial follow-up
spectroscopic observations. About half of the newly detected massive stars are
found outside the known clusters (Arches, Quintuplet, and Central). Many
previously known diffuse thermal features are now resolved into arrays of
intriguingly fine linear filaments indicating a profound role of magnetic
fields in sculpting the gas. The bright spiral-like Paschen-alpha emission
around Sgr A* is seen to be well confined within the known dusty torus. In the
directions roughly perpendicular to it, we further detect faint, diffuse
Paschen-alpha emission features, which, like earlier radio images, suggest an
outflow from the structure. In addition, we detect various compact
Paschen-alpha nebulae, probably tracing the accretion and/or ejection of stars
at various evolutionary stages.Comment: accepted for publication in MNRAS; a version of higher resolution
images may be found at http://www.astro.umass.edu/~wqd/papers/hst/paper1.pd
Results from a Near Infrared Search for Emission-line Stars in the Inner Galaxy: Spectra of New Wolf-Rayet Stars
We present follow-up spectroscopy of emission line candidates detected on
near-infrared narrow band images in the inner Galaxy (Homeier et al. 2003). The
filters are optimized for the detection of Wolf-Rayet stars and other objects
which exhibit emission--lines in the 2 m region. Approximately three
square degrees along the Galactic plane have been analyzed in seven
narrow--filters (four emission--lines and three continuum). We have discovered
4 new Wolf-Rayet stars and present coordinates, finding charts, and K-band
spectra.Comment: To appear in Astronomy & Astrophysic
Massive Young Stellar Objects in the Galactic Center. I. Spectroscopic Identification from Spitzer/IRS Observations
We present results from our spectroscopic study, using the Infrared
Spectrograph (IRS) onboard the Spitzer Space Telescope, designed to identify
massive young stellar objects (YSOs) in the Galactic Center (GC). Our sample of
107 YSO candidates was selected based on IRAC colors from the high spatial
resolution, high sensitivity Spitzer/IRAC images in the Central Molecular Zone
(CMZ), which spans the central ~300 pc region of the Milky Way Galaxy. We
obtained IRS spectra over 5um to 35um using both high- and low-resolution IRS
modules. We spectroscopically identify massive YSOs by the presence of a 15.4um
shoulder on the absorption profile of 15um CO2 ice, suggestive of CO2 ice mixed
with CH3OH ice on grains. This 15.4um shoulder is clearly observed in 16
sources and possibly observed in an additional 19 sources. We show that 9
massive YSOs also reveal molecular gas-phase absorption from CO2, C2H2, and/or
HCN, which traces warm and dense gas in YSOs. Our results provide the first
spectroscopic census of the massive YSO population in the GC. We fit YSO models
to the observed spectral energy distributions and find YSO masses of 8 - 23
Msun, which generally agree with the masses derived from observed radio
continuum emission. We find that about 50% of photometrically identified YSOs
are confirmed with our spectroscopic study. This implies a preliminary star
formation rate of ~0.07 Msun/yr at the GC.Comment: Accepted for publication in Ap
The Stellar Initial Mass Function in the Galactic Center
Massive stars define the upper limits of the star formation process, dominate
the energetics of their local environs, and significantly affect the chemical
evolution of galaxies. Their role in starburst galaxies and the early Universe
is likely to be important, but we still do not know the maximum mass that a
star can possess, i.e.``the upper mass cutoff.'' I will discuss results from a
program to measure the upper mass cutoff and IMF slope in the Galactic Center.
The results suggest that the IMF in the Galactic center may deviate
significantly from the Salpeter value, and that there may be an upper mass
cutoff to the initial mass function of 150 Msun.Comment: To be published in the IMF@50 conference proceeding
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