We present high spatial resolution (5".1 x 2".7) Owens Valley Radio Observatory (OVRO) millimeter array observations of HCN (J = 1-0) and HCO(+) (J = 1-0) emission in the inner 3 pc of the Galaxy ...(0.04 pc 6 1"). The HCN and HCO(+) emission of the circumnuclear disk (CND) is distributed in a well-defined ring with a peak at a radius of 1.6 pc. The observed radial velocities are generally consistent with rotation at 6110 km s(-1) (except along the western edge of the CND). The HCO(+)/HCN emission ratio is typically 60.4 but with significant variations. The variations in the HCO(+)/HCN emission and absorption ratios can be attributed to greater abundances of HCO(+) in lower density regions both within the CND and along the line of sight. The HCN emission is well correlated with the H(2) emission at 2.12 km both in the main emission lobes of the CND and also in four H(2) and HCN filaments. Multiple areas of interaction between the ionized gas and the CND are also seen; the western arm of the minispiral is spatially and kinematically consistent with being the ionized inner edge of the CND, and the northern arm may be connected to the CND northeastern extension. With the enhanced spatial resolution of the HCN maps, we resolve numerous dense molecular gas cores within the CND with characteristic diameter 67" (0.25 pc). For 26 of the more isolated cores, we have measured sizes, velocity widths, and integrated fluxes. From these properties we estimated three masses for each core: a virial mass assuming the cores are gravitationally bound, an optically thick mass from observed column densities of HCN, and a lower limit mass assuming the HCN emission is optically thin and shock excitation is negligible. The virial and optically thick masses are in good agreement with a typical mass of (2-3) x 10(4) M( and a total CND gas mass of 10(6) M( . The internal densities implied by these core masses (assuming a uniform density distribution for each core) are on average (3-4) x 10(7) cm(-3). The core densities are high enough to be stable against tidal disruption from Sgr A* and the central stellar concentration. This tidal stability suggests a longer lifetime for the CND. The high densities and masses within the cores might support star formation either in the CND itself or within a core infalling toward the inner parsec, thus providing a mechanism for the formation of the young stellar population observed in the inner arcseconds of the Galaxy.
High spatial resolution observations in the 1 to 3.5 μm region of the Galactic Center source known historically as IRS 13 are presented. They include ground-based adaptive optics images in the H, Kp ...(2.12/0.4 μm) and L bands, HST-NICMOS data in filters between 1.1 and 2.2 μm, and integral field spectroscopic data from BEAR, an Imaging FTS, in the $\ion{He}{i}$ 2.06 μm and the Brγ line regions. Analysis of all these data provides a completely new picture of the main component, IRS 13E, which appears as a cluster of seven individual stars within a projected diameter of ~0.5´´ (0.02 pc). The brightest sources, 13E1, 13E2, 13E3 which is detected as a binary, and 13E4, are all massive stars of different type. The star 13E1 is a luminous, blue object, with no detected emission line. 13E2 and 13E4 are two hot, high-mass emission line stars, 13E2 being at the WR stage and 13E4 a massive O-type star. In contrast, 13E3A and B are extremely red objects, proposed as other examples of dusty WR stars, like IRS 21 (Tanner et al. CITE). All these sources have a common westward proper motion (Ott et al. CITE) indicating they are bounded. Two other sources, detected after deconvolution of the AO images in the H and Kp bands, are also identified. One, that we call 13E5, is a red source similar to 13E3A and B, while the other one, 13E6, is probably a main sequence O star in front of the cluster. Considering this exceptional concentration of comoving massive hot stars, IRS 13E is proposed as the remaining core of a massive star cluster, which could harbor an intermediate-mass black hole (IMBH) (Portegies Zwart & McMillan CITE) of ~1300 $M_{\odot}$. This detection plays in favor of a scenario, first suggested by Gerhard (CITE), in which the helium stars and the other hot stars in the central parsec originate from the stripping of a massive cluster formed several tens of pc from the center. This cluster would have spiraled towards SgrA$^{\star}$, and IRS 13E would be its remnant. Furthermore, IRS 13E might be the second black hole needed according to a model by Hansen & Milosavljević (CITE) to drag massive main-sequence stars, in the required timescale, very close to the massive black hole. The detection of a discrete X-ray emission (Baganoff et al. CITE) at the IRS 13 position (within the positional accuracy) is examined in this context.
The GLIMPSE (Galactic Legacy Mid-Plane Survey Extraordinaire) Point Source Catalog of 630 million mid-infrared sources toward the inner Galaxy, 10 , "l" 65 , 65 and "b" , 1, was used to determine the ...distribution of stars in Galactic longitude, l, latitude, b, and apparent magnitude, m. The counts versus longitude can be approximated by the modified Bessel function N = N sub(0)(l/l sub(0)) K sub(1)(l/l sub(0)), where l sub(0) is insensitive to limiting magnitude, band choice, and side of Galactic center: l sub(0) = 17-30 with a best-fit value in the 4.5 km band of l sub(0) = 24 c 4. Modeling the source distribution as an exponential disk yields a radial scale length of H sub(*) = 3.9 c 0.6 kpc. There is a pronounced north-south asymmetry in source counts for "l" 30, with 625% more stars in the north. For l = 10-30, there is a strong enhancement of stars of m = 11.5-13.5 mag. A linear bar passing through the Galactic center with half-length R sub(bar) = 4.4 c 0.5 kpc, tilted by h = 44 c 10 to the Sun-Galactic center line, provides the simplest interpretation of these data. We examine the possibility that enhanced source counts at l = 26-28, 31.5-34, and 306-309 are related to Galactic spiral structure. Total source counts are depressed in regions where the counts of red objects (m sub(K)-m sub(8.0) > 3) peak. In these areas, the counts are reduced by extinction due to molecular gas, high diffuse backgrounds associated with star formation, or both.
We present new identifications of infrared counterparts to the population of hard X-ray sources near the Galactic center detected by the Chandra X-ray Observatory. We have spectroscopically confirmed ...16 new massive stellar counterparts to the X-ray population, including nitrogen-type (WN) and carbon-type (WC) Wolf-Rayet stars, and O supergiants. These discoveries increase the total sample of massive stellar X-ray sources in the Galactic center region to 30 (possibly 31). For the majority of these sources, the X-ray photometry is consistent with thermal emission from plasma having temperatures in the range of kT = 1-8 keV or non-thermal emission having power-law indices in the range of -1 {approx}< GAMMA {approx}< 3, and X-ray luminosities in the range of L{sub X} {approx} 10{sup 32}-10{sup 34} erg s{sup -1} (0.5-8.0 keV). Several sources have exhibited X-ray variability of several factors between observations. These X-ray properties are not a ubiquitous feature of single massive stars but are typical of massive binaries, in which the high-energy emission is generated by the collision of supersonic winds, or by accretion onto a compact companion. However, without direct evidence for companions, the possibility of intrinsic hard X-ray generation from single stars cannot be completely ruled out. The spectral energy distributions of these sources exhibit significant infrared excess, attributable to free-free emission from ionized stellar winds, supplemented by hot dust emission in the case of the WC stars. With the exception of one object located near the outer regions of the Quintuplet cluster, most of the new stars appear isolated or in loose associations. Seven hydrogen-rich WN and O stars are concentrated near the Sagittarius B H II region, while other similar stars and more highly evolved hydrogen-poor WN and WC stars lie scattered within {approx}50 pc, in projection, of Sagitarrius A West. We discuss various mechanisms capable of generating the observed X-rays and the implications these stars have for massive star formation in the Galaxy's Central Molecular Zone.
We present a catalog of 9017 X-ray sources identified in Chandra observations of a 2°X 08 field around the Galactic center. This enlarges the number of known X-ray sources in the region by a factor ...of 2.5. The catalog incorporates all of the ACIS-I observations as of 2007 August, which total 2.25 Ms of exposure. At the distance to the Galactic center (8 kpc), we are sensitive to sources with luminosities of 4 X 1032 erg s-1 (0.5-8.0 keV; 90% confidence) over an area of 1 deg2, and up to an order of magnitude more sensitive in the deepest exposure (1.0 Ms) around Sgr A*. The positions of 60% of our sources are accurate to <1 '' (95% confidence), and 20% have positions accurate to <05. We search for variable sources, and find that 3% exhibit flux variations within an observation, and 10% exhibit variations from observation-to-observation. We also find one source, CXOUGC J174622.7 - 285218, with a periodic 1745 s signal (1.4% chance probability), which is probably a magnetically accreting cataclysmic variable. We compare the spatial distribution of X-ray sources to a model for the stellar distribution, and find 2.8s evidence for excesses in the numbers of X-ray sources in the region of recent star formation encompassed by the Arches, Quintuplet, and Galactic center star clusters. These excess sources are also seen in the luminosity distribution of the X-ray sources, which is flatter near the Arches and Quintuplet than elsewhere in the field. These excess point sources, along with a similar longitudinal asymmetry in the distribution of diffuse iron emission that has been reported by other authors, probably have their origin in the young stars that are prominent at l 01.
We determine and tabulate A sub( lambda )/A sub(K), the wavelength dependence of interstellar extinction, in the Galactic plane for 1.25 mu m less than or equal to lambda less than or equal to mu 8.0 ...mu m along two lines of sight: l = 42 degree and 284 degree . The first is a relatively quiescent and unremarkable region; the second contains the giant H II region RCW 49, as well as a "field" region unrelated to the cluster and nebulosity. Areas near these Galactic longitudes were imaged at J, H, and K bands by 2MASS and at 3-8 mu m by Spitzer for the GLIMPSE Legacy program. We measure the mean values of the color excess ratios (A sub( lambda ) - A sub(K))/(A sub(J) - A sub(K)) directly from the color distributions of observed stars. The extinction ratio between two of the filters, e.g., A sub(J)/A sub(K), is required to calculate A sub( lambda )/A sub(K) from those measured ratios. We use the apparent JHK magnitudes of giant stars along our two sight lines and fit the reddening as a function of magnitude (distance) to determine A sub(J)kpc super(-1), A sub(K)kpc super(-1), and A sub(J)/A sub(K). Our values of A sub( lambda )/A sub(K) show a flattening across the 3-8 mu m wavelength range, roughly consistent with the extinction measurements derived by Lutz and coworkers for the sight line toward the Galactic center.
We have obtained Spitzer IRAC observations of the central 2.0 degree x 1.4 degree ( similar to 280 x 200 pc) of the Galaxy at 3.6-8.0 mu m. A point-source catalog of 1,065,565 objects is presented. ...The catalog includes magnitudes for the point sources at 3.6, 4.5, 5.8, and 8.0 mu m, as well as JHK sub(S) photometry from Two Micron All Sky Survey (2MASS). The point-source catalog is confusion limited with average limits of 12.4, 12.1, 11.7, and 11.2 mag for 3.6, 4.5, 5.8, and 8.0, respectively. We find that the confusion limits are spatially variable because of stellar surface density, background surface brightness level, and extinction variations across the survey region. The overall distribution of point-source density with Galactic latitude and longitude is essentially constant, but structure does appear when sources of different magnitude ranges are selected. Bright stars show a steep decreasing gradient with Galactic latitude and a slow decreasing gradient with Galactic longitude, with a peak at the position of the Galactic center. From IRAC color-magnitude and color-color diagrams, we conclude that most of the point sources in our catalog have IRAC magnitudes and colors characteristic of red giant and asymptotic giant branch (AGB) stars.
ABSTRACT
We report the discovery of 19 hot, evolved, massive stars near the Galactic center region (GCR). These objects were selected for spectroscopy owing to their detection as strong sources of ...Paschen-α (Pα) emission-line excess, following a narrowband imaging survey of the central 0.°65 × 0.°25 (
l
,
b
) around Sgr A* with the
Hubble Space Telescope
. Discoveries include six carbon-type (WC) and five nitrogen-type (WN) Wolf–Rayet stars, six O supergiants, and two B supergiants. Two of the O supergiants have X-ray counterparts having properties consistent with solitary O stars and colliding-wind binaries. The infrared photometry of 17 stars is consistent with the Galactic center distance, but 2 of them are located in the foreground. Several WC stars exhibit a relatively large infrared excess, which is possibly thermal emission from hot dust. Most of the stars appear scattered throughout the GCR, with no relation to the three known massive young clusters; several others lie near the Arches and Quintuplet clusters and may have originated within one of these systems. The results of this work bring the total sample of Wolf–Rayet (WR) stars in the GCR to 88. All sources of strong Pα excess have been identified in the area surveyed with
HST
, which implies that the sample of WN stars in this region is near completion, and is dominated by late (WNL) types. The current WC sample, although probably not complete, is almost exclusively dominated by late (WCL) types. The observed WR subtype distribution in the GCR is a reflection of the intrinsic rarity of early subtypes (WNE and WCE) in the inner Galaxy, an effect that is driven by metallicity.
The Bubbling Galactic Disk Churchwell, E; Povich, M. S; Allen, D ...
Astrophysical journal/The Astrophysical journal,
10/2006, Letnik:
649, Številka:
2
Journal Article
Recenzirano
Odprti dostop
A visual examination of the images from the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) has revealed 322 partial and closed rings that we propose represent partially or fully ...enclosed three-dimensional bubbles. We argue that the bubbles are primarily formed by hot young stars in massive star formation regions. We have found an average of about 1.5 bubbles per square degree. About 25% of the bubbles coincide with known radio H II regions, and about 13% enclose known star clusters. It appears that B4-B9 stars (too cool to produce detectable radio H II regions) probably produce about three-quarters of the bubbles in our sample, and the remainder are produced by young O-B3 stars that produce detectable radio H II regions. Some of the bubbles may be the outer edges of H II regions where PAH spectral features are excited and may not be dynamically formed by stellar winds. Only three of the bubbles are identified as known SNRs. No bubbles coincide with known planetary nebulae or W-R stars in the GLIMPSE survey area. The bubbles are small. The distribution of angular diameters peaks between 1' and 3' with over 98% having angular diameters less than 10' and 88% less than 4'. Almost 90% have shell thicknesses between 0.2 and 0.4 of their outer radii. Bubble shell thickness increases approximately linearly with shell radius. The eccentricities are rather large, peaking between 0.6 and 0.7; about 65% have eccentricities between 0.55 and 0.85.
PDF
