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Register a new userNear-infrared spectroscopy in NGC 7538
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The characterisation of the stellar population toward young high-mass star-forming regions allows to constrain fundamental cluster properties like distance and age. These are essential when using high-mass clusters as probes to conduct Galactic studies. NGC 7538 is a star-forming region with an embedded stellar population only unearthed in the near-infrared. We present the first near-infrared spectro-photometric study of the candidate high-mass stellar content in NGC 7538. We obtained H and K spectra of 21 sources with both the multi-object and long-slit modes of LIRIS at the WHT, and complement these data with sub-arcsecond JHKs photometry of the region using the imaging mode of the same instrument. We find a wide variety of objects within the studied stellar population of NGC 7538. Our results discriminate between a stellar population associated to the HII region, but not contained within its extent, and several pockets of more recent star formation. We report the detection of CO bandhead emission toward several sources as well as other features indicative of a young stellar nature. We infer a spectro-photometric distance of 2.7+-0.5 kpc, an age spread in the range 0.5-2.2 Myr and a total mass ~1.7x10^3 Msun for the older population.
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We present high resolution (R = 75,000-100,000) mid-infrared spectra of the high-mass embedded young star IRS 1 in the NGC 7538 star-forming region. Absorption lines from many rotational states of C2H2, 13C12CH2, CH3, CH4, NH3, HCN, HNCO, and CS are seen. The gas temperature, column density, covering factor, line width, and Doppler shift for each molecule are derived. All molecules were fit with two velocity components between -54 and -63 km/s. We find high column densities (~ 10e16 cm^2) for all the observed molecules compared to values previously reported and present new results for CH3 and HNCO. Several physical and chemical models are considered. The favored model involves a nearly edge-on disk around a massive star. Radiation from dust in the inner disk passes through the disk atmosphere, where large molecular column densities can produce the observed absorption line spectrum.
We present high resolution images of NGC 2071-IR in the $J$, $H$, and $K$ bands and in the emission at 2.12 $mu$m of the v=$1-0$ $S$(1) line of molecular hydrogen. We also present moderate resolution K-band spectra of two young stellar objects, IRS 1 and IRS 3, within NGC 2071-IR, that are candidates sources of one or more of the outflows observed in the region. Two of the eight originally identified infrared point sources in NGC 2071-IR are binaries, and we identifiy two new sources, one coincident with the radio source VLA-1 and highly reddened. The H2 $Q$(3)/$S$(1) line intensity ratios at IRS 1 and IRS 3 yield high and very high extinctions, respectively, to them, as is implied by their near-infrared colors and K-band continuum slopes. The spectra also reveal the presence of hot, dense circumstellar molecular gas in each, suggesting that both are strong candidates for having energetic molecular outflows. We agree with a previous suggestion that IRS 1 is the likely source of an E-W-oriented outflow and conclude that this outflow is probably largely out of the plane of the sky. We also conclude that if IRS 3 is the source of the large scale NE-SW outflow, as has been previously suggested, its jet/wind must precess in order to explain the angular width of that outflow. We discuss the natures of the point sources and their probable contributions, if any, to the complex morphology of the H2 line emission.
We investigate the deuteration of methanol towards the high-mass star forming region NGC 7538-IRS1. We have carried out a multi-transition study of CH$_3$OH, $^{13}$CH$_3$OH and of the deuterated fllavors, CH$_2$DOH and CH$_3$OD, between 1.0--1.4 mm with the IRAM-30~m antenna. In total, 34 $^{13}$CH$_3$OH, 13 CH$_2$DOH lines and 20 CH$_3$OD lines spanning a wide range of upper-state energies (E$_{up}$) were detected. From the detected transitions, we estimate that the measured D/H does not exceed 1$%$, with a measured CH$_2$DOH/CH$_3$OH and CH$_3$OD/CH$_3$OH of about (32$pm$8)$times$10$^{-4}$ and (10$pm$4)$times$10$^{-4}$, respectively. This finding is consistent with the hypothesis of a short-time scale formation during the pre-stellar phase. We find a relative abundance ratio CH$_2$DOH/CH$_3$OD of 3.2 $pm$ 1.5. This result is consistent with a statistical deuteration. We cannot exclude H/D exchanges between water and methanol if water deuteration is of the order 0.1$%$, as suggested by recent Herschel observations.
Context: The Asymptotic Giant Branch (AGB) phase is characterised by substantial mass loss that is accompanied by the formation of dust. In extreme cases this will make the star no longer visible in the optical. For a better understanding of AGB evolution it is important to identify and characterise these very red AGB stars. Aims: The first aim of this article is to improve the census of red AGB stars in three Local Group galaxies, based on near-IR spectroscopic observations of new candidates with red 2MASS (J-K) colours. The opportunity is taken to compare the near-IR spectra with those of Milky Way stars. Methods: We used ISAAC on the ESO VLT to take J and H-band spectra of 36 targets in Fornax, Sculptor and NGC 6822. Results: Twelve new C-stars are found in Fornax, and one is confirmed in Sculptor. All C-stars have (J-K) > 1.6, and are brighter than -3.55 in bolometric magnitude. Ten new oxygen-rich late-type giant stars are identified in Fornax, but none is extremely red or very luminous. Five luminous O-rich AGB stars are identified in NGC 6822, of which 3 show water absorption, indicative of spectral type M. Again, none is as red as Milky Way OH/IR stars, but in this galaxy the list of candidate AGB stars is biased against very red objects. In some C-stars with (J-K)>2 an extremely strong 1.53 $mu$m absorption band is found. These stars are probably all Mira variables and the feature is related to the low temperature, high density chemistry that is a first step towards dust formation and mass loss.
We conducted systematic observations of the HI Br-alpha line (4.05 micron) and the polycyclic aromatic hydrocarbon (PAH) feature (3.3 micron) in 50 nearby (z<0.3) ultraluminous infrared galaxies (ULIRGs) with AKARI. The Br-alpha line is predicted to be the brightest among the HI lines under high dust-extinction conditions (A_V>15 mag). The Br-alpha line traces ionizing photons from OB stars and so is used as an indicator of star formation on the assumption of the initial mass function. We detected the Br-alpha line in 33 ULIRGs. The luminosity of the line (L_BrA) correlates well with that of the 3.3 micron PAH emission (L_3.3). Thus we utilize L_3.3 as an indicator of star formation in fainter objects where the Br-alpha line is undetected. The mean L_BrA/L_IR ratio in LINERs/Seyferts is significantly lower than that in HII galaxies. This difference is reconfirmed with the L_3.3/L_IR ratio in the larger sample (46 galaxies). Using the ratios, we estimate that the contribution of starburst in LINERs/Seyferts is ~67%, and active galactic nuclei contribute to the remaining ~33%. However, comparing the number of ionizing photons, Q_BrA, derived from L_BrA with that, Q_IR, expected from star formation rate required to explain L_IR, we find that the mean Q_BrA/Q_IR ratio is only 55.5+/-7.5% even in HII galaxies which are thought to be energized by pure starburst. This deficit of ionizing photons traced by the Br-alpha line is significant even taking heavy dust extinction into consideration. We propose that dust within HII regions absorbs a significant fraction of ionizing photons.
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