Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userSurveying the Giant HII Regions of the Milky Way with SOFIA: II. M17
69
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We present our second set of results from our mid-infrared imaging survey of Milky Way Giant HII regions. We used the FORCAST instrument on the Stratospheric Observatory For Infrared Astronomy to obtain 20 and 37$mu$m images of the central ~10X10 area of M17. We investigate the small- and large-scale properties of M17 using our data in conjunction with previous multi-wavelength observations. The spectral energy distributions of individual compact sources were constructed with Spitzer-IRAC, SOFIA-FORCAST, and Herschel-PACS photometry data and fitted with massive young stellar object (MYSO) models. Seven sources were found to match the criteria for being MYSO candidates, four of which are identified here for the first time, and the stellar mass of the most massive object, UC1, is determined to be 64 solar mass. We resolve the extended mid-infrared emission from the KW Object, and suggest that the angle of this extended emission is influenced by outflow. IRS5 is shown to decrease in brightness as a function of wavelength from the mid- to far-infrared, and has several other indicators that point to it being an intermediate mass Class II object and not a MYSO. We find that the large-scale appearance of emission in M17 at 20$mu$m is significantly affected by contamination from the [SIII] emission line from the ionized gas of the Giant HII region. Finally, a number of potential evolutionary tracers yield a consistent picture suggesting that the southern bar of M17 is likely younger than the northern bar.
rate research
Read More
Filamentary structures are common morphological features of the cold, molecular interstellar medium (ISM). Recent studies have discovered massive, hundred-parsec-scale filaments that may be connected to the large-scale, Galactic spiral arm structure. Addressing the nature of these Giant Molecular Filaments (GMFs) requires a census of their occurrence and properties. We perform a systematic search of GMFs in the fourth Galactic quadrant and determine their basic physical properties. We identify GMFs based on their dust extinction signatures in near- and mid-infrared and velocity structure probed by ^{13}CO line emission. We use the ^{13}CO line emission and ATLASGAL dust emission data to estimate the total and dense gas masses of the GMFs. We combine our sample with an earlier sample from literature and study the Galactic environment of the GMFs. We identify nine GMFs in the fourth Galactic quadrant; six are located in the Centaurus spiral arm and three in inter-arm regions. Combining this sample with an earlier study using the same identification criteria in the first Galactic quadrant results in 16 GMFs, nine of which are located within spiral arms. The GMFs have sizes of 80-160 pc and ^{13}CO-derived masses between 5-90 x 10^{4} Msun. Their dense gas mass fractions are between 1.5-37%, being higher in the GMFs connected to spiral arms. We also compare the different GMF-identification methods and find that emission and extinction based techniques overlap only partially, highlighting the need to use both to achieve a complete census.
The metallicity structure of the Milky Way disk stems from the chemodynamical evolutionary history of the Galaxy. We use the National Radio Astronomy Observatory Karl G. Jansky Very Large Array to observe ~8-10 GHz hydrogen radio recombination line and radio continuum emission toward 82 Galactic HII regions. We use these data to derive the electron temperatures and metallicities for these nebulae. Since collisionally excited lines from metals (e.g., oxygen, nitrogen) are the dominant cooling mechanism in HII regions, the nebular metallicity can be inferred from the electron temperature. Including previous single dish studies, there are now 167 nebulae with radio-determined electron temperature and either parallax or kinematic distance determinations. The interferometric electron temperatures are systematically 10% larger than those found in previous single dish studies, likely due to incorrect data analysis strategies, optical depth effects, and/or the observation of different gas by the interferometer. By combining the interferometer and single dish samples, we find an oxygen abundance gradient across the Milky Way disk with a slope of -0.052 +/- 0.004 dex/kpc. We also find significant azimuthal structure in the metallicity distribution. The slope of the oxygen gradient varies by a factor of ~2 when Galactocentric azimuths near 30 deg are compared with those near 100 deg. This azimuthal structure is consistent with simulations of Galactic chemodynamical evolution influenced by spiral arms.
High-mass stars form in much richer environments than those associated with isolated low-mass stars, and once they reach a certain mass, produce ionised (HII) regions. The formation of these pockets of ionised gas are unique to the formation of high-mass stars (M $>8$ M$_odot$), and present an excellent opportunity to study the final stages of accretion, which could include accretion through the HII region itself. This study of the dynamics of the gas on both sides of these ionisation boundaries in very young HII regions aims to quantify the relationship between the HII regions and their immediate environments.We present high-resolution ($sim$ 0.5$$) ALMA observations of nine HII regions selected from the Red MSX Source (RMS) survey with compact radio emission and bolometric luminosities greater than 10$^4$ L$_odot$. We focus on the initial presentation of the data, including initial results from the radio recombination line H29$alpha$, some complementary molecules, and the 256 GHz continuum emission. Of the six (out of nine) regions with H29$alpha$ detections, two appear to have cometary morphologies with velocity gradients across them, and two appear more spherical with velocity gradients suggestive of infalling ionised gas. The remaining two were either observed at low resolution or had signals that were too weak to draw robust conclusions. We also present a description of the interactions between the ionised and molecular gas (as traced by CS (J=5-4)), often (but not always) finding theHII region had cleared its immediate vicinity of molecules. Of our sample of nine, the observations of the two clusters expected to have the youngest HII regions (from previous radio observations) are suggestive of having infalling motions in the H29$alpha$ emission, which could be indicative of late stage accretion onto the stars despite the presence of an HII region.
The Milky Ways metal-poor stars are nearby ancient objects that are used to study early chemical evolution and the assembly and structure of the Milky Way. Here we present reliable metallicities of $sim280,000$ stars with $-3.75 lesssim$ [Fe/H] $lesssim -0.75$ down to $g=17$ derived using metallicity-sensitive photometry from the second data release (DR2) of the SkyMapper Southern Survey. We use the dependency of the flux through the SkyMapper $v$ filter on the strength of the Ca II K absorption features, in tandem with SkyMapper $u,g,i$ photometry, to derive photometric metallicities for these stars. We find that metallicities derived in this way compare well to metallicities derived in large-scale spectroscopic surveys, and use such comparisons to calibrate and quantify systematics as a function of location, reddening, and color. We find good agreement with metallicities from the APOGEE, LAMOST, and GALAH surveys, based on a standard deviation of $sigmasim0.25$dex of the residuals of our photometric metallicities with respect to metallicities from those surveys. We also compare our derived photometric metallicities to metallicities presented in a number of high-resolution spectroscopic studies to validate the low metallicity end ([Fe/H] $< -2.5$) of our photometric metallicity determinations. In such comparisons, we find the metallicities of stars with photometric [Fe/H] $< -2.5$ in our catalog show no significant offset and a scatter of $sigmasim$0.31dex level relative to those in high-resolution work when considering the cooler stars ($g-i > 0.65$) in our sample. We also present an expanded catalog containing photometric metallicities of $sim720,000$ stars as a data table for further exploration of the metal-poor Milky Way.
We apply the spectroscopy-based stellar-color regression (SCR) method to perform an accurate photometric re-calibration of the second data release from the SkyMapper Southern Survey (SMSS DR2). From comparison with a sample of over 200,000 dwarf stars with stellar atmospheric parameters taken from GALAH+ DR3 and with accurate, homogeneous photometry from $Gaia$ DR2, zero-point offsets are detected in the original photometric catalog of SMSS DR2, in particular for the gravity- and metallicity-sensitive $uv$ bands. For $uv$ bands, the zero-point offsets are close to zero at very low extinction, and then steadily increase with $E (B - V)$, reaching as large as 0.174 and 0.134 mag respectively, at $E (B - V) sim 0.5$ mag. These offsets largely arise from the adopted dust term in the transformations used by SMSS DR2 to construct photometric calibrators from the ATLAS reference catalog. For the $gr$ bands, the zero-point offsets exhibit negligible variations with SFD $E(B - V )$, due to their tiny coefficients on the dust term in the transformation. Our study also reveals small, but significant, spatial variations of the zero-point offsets in all $uvgr$ bands. External checks using Stromgren photometry, WD loci and the SDSS Stripe 82 standard-star catalog independently confirm the zero-points found by our revised SCR method.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
