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Register a new userVLT-ISAAC 3-5 micron spectroscopy of embedded young low-mass stars. III. Intermediate-mass sources in Vela
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W. F. Thi
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We performed a spectroscopic survey toward five intermediate-mass class I YSOs located in the Southern Vela molecular cloud in the L and M bands at resolving powers 600-800 up to 10,000, using the Infrared Spectrometer and Array Camera mounted on the VLT-ANTU. Lower mass companion objects were observed simultaneously in both bands. Solid H2O at 3 micron is detected in all sources, including the companion objects. CO ice at 4.67 micron is detected in a few main targets and one companion object. One object (LLN 19) shows little CO ice but strong gas-phase CO ro-vibrational lines in absorption. The CO ice profiles are different from source to source. The amount of water ice and CO ice trapped in a water-rich mantle may correlate with the flux ratio at 12 and 25 micron. The abundance of H2O-rich CO likely correlates with that of water ice. A weak feature at 3.54 mu attributed to solid CH3OH and a broad feature near 4.62 mu are observed toward LLN17, but not toward the other sources. The derived abundances of solid CH3OH and OCN- are ~10% and ~1% of the H2O ice abundance respectively. The H2O optical depths do not show an increase with envelope mass, nor do they show lower values for the companion objects compared with the main protostar. The line-of-sight CO ice abundance does not correlate with the source bolometric luminosity. Comparison of the solid CO profile toward LLN17, which shows an extremely broad CO ice feature, and that of its lower mass companion at a few thousand AU, which exhibits a narrow profile, together with the detection of OCN- toward LLN17 provide direct evidences for local thermal processing of the ice.
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The 4.62$mu$m (2164.5 cm$^{-1}$) `XCN band has been detected in the $M$-band spectra of 34 deeply embedded young stellar objects (YSOs), observed with high signal-to-noise and high spectral resolution with the VLT-ISAAC spectrometer, providing the first opportunity to study the solid OCN$^-$ abundance toward a large number of low-mass YSOs. It is shown unequivocally that at least two components, centred at 2165.7 cm$^{-1}$ (FWHM = 26 cm$^{-1}$) and 2175.4 cm$^{-1}$ (FWHM = 15 cm$^{-1}$), underlie the XCN band. Only the 2165.7-component can be ascribed to OCN$^-$, embedded in a strongly hydrogen-bonding, and possibly thermally annealed, ice environment based on laboratory OCN$^-$ spectra. In order to correct for the contribution of the 2175.4-component to the XCN band, a phenomenological decomposition into the 2165.7- and the 2175.4-components is used to fit the full band profile and derive the OCN$^-$ abundance for each line-of-sight. The same analysis is performed for 5 high-mass YSOs taken from the ISO-SWS data archive. Inferred OCN$^-$ abundances are $leq$ 0.85 % toward low-mass YSOs and $leq$ 1 % toward high-mass YSOs, except for W33 A. Abundances are found to vary by at least a factor of 10--20 and large source-to-source abundance variations are observed within the same star-forming cloud complex on scales down to 400 AU, indicating that the OCN$^-$ formation mechanism is sensitive to local conditions. The inferred abundances allow quantitatively for photochemical formation of OCN$^-$, but the large abundance variations are not easily explained in this scenario unless local radiation sources or special geometries are invoked. Surface chemistry should therefore be considered as an alternative formation mechanism.
Massive clumps, prior to the formation of any visible protostars, are the best candidates to search for the elusive massive starless cores. In this work we investigate the dust and gas properties of massive clumps selected to be 70 micron quiet, therefore good starless candidates. Our sample of 18 clumps has masses 300 < M < 3000 M_sun, radius 0.54 < R < 1.00 pc, surface densities Sigma > 0.05 g cm^-2 and luminosity/mass ratio L/M < 0.3. We show that half of these 70 micron quiet clumps embed faint 24 micron sources. Comparison with GLIMPSE counterparts shows that 5 clumps embed young stars of intermediate stellar mass up to ~5.5 M_sun. We study the clump dynamics with observations of N2H+ (1-0), HNC (1-0) and HCO+ (1-0) made with the IRAM 30m telescope. Seven clumps have blue-shifted spectra compatible with infall signatures, for which we estimate a mass accretion rate 0.04 < M_dot < 2.0 x 10^-3 M_sun yr^-1, comparable with values found in high-mass protostellar regions, and free-fall time of the order of t_ff = 3 x 10^5 yr. The only appreciable difference we find between objects with and without embedded 24 micron sources is that the infall rate appears to increase from 24 micron dark to 24 micron bright objects. We conclude that all 70 micron quiet objects have similar properties on clump scales, independently of the presence of an embedded protostar. Based on our data we speculate that the majority, if not all of these clumps may already embed faint, low-mass protostellar cores. If these clumps are to form massive stars, this must occur after the formation of these lower mass stars.
Near IR spectra obtained with ISAAC at VLT, have been used to pose constraints on the evolutionary state and accretion properties of a sample of five embedded YSOs located in the R CrA core. This sample includes three Class I sources (HH100 IR, IRS2 and IRS5), and two sources with NIR excesses (IRS6 and IRS3). Absorption lines have been detected in the medium resolution spectra of all the observed targets, together with emission lines likely originating in the disk-star-wind connected regions. We derived spectral types, veiling and stellar luminosity of the five observed sources, which in turn have been used to infer their mass and age adopting pre-main sequence evolutionary tracks. We find that in HH100 IR and IRS2 most of the bolometric luminosity is due to accretion, while the other three investigated sources, including the Class I object IRS5a, present a low accretion activity (L_{acc}/L_{bol} < 0.2). We observe a general correlation between the accretion luminosity, the IR veiling and the emission line activity of the sources. A correlation between the accretion activity and the spectral energy distribution slope is recognizable but with the notable exception of IRS5a. Our analysis therefore shows how the definition of the evolutionary stage of deeply embedded YSOs by means of IR colors needs to be more carefully refined.
Abridged: Observed abundances of extremely metal-poor (EMP) stars in the Halo hold clues for the understanding of the ancient universe. Interpreting these clues requires theoretical stellar models at the low-Z regime. We provide the nucleosynthetic yields of intermediate-mass Z=$10^{-5}$ stars between 3 and 7.5 $M_{sun}$, and quantify the effects of the uncertain wind rates. We expect these yields can be eventually used to assess the contribution to the chemical inventory of the early universe, and to help interpret abundances of selected C-enhanced EMP stars. By comparing our models and other existing in the literature, we explore evolutionary and nucleosynthetic trends with wind prescriptions and with initial metallicity. We compare our results to observations of CEMP-s stars belonging to the Halo. The yields of intermediate-mass EMP stars reflect the effects of very deep second dredge-up (for the most massive models), superimposed with the combined signatures of hot-bottom burning and third dredge-up. We confirm the reported trend that models with initial metallicity Z$_{ini}$ <= 0.001 give positive yields of $^{12}C, ^{15}N, ^{16}O$, and $^{26}Mg$. The $^{20}Ne, ^{21}Ne$, and $^{24}Mg$ yields, which were reported to be negative at Z$_{ini}$ = 0.0001, become positive for Z=$10^{-5}$. The results using two different prescriptions for mass-loss rates differ widely in terms of the duration of the thermally-pulsing (Super) AGB phase, overall efficiency of the third dredge-up episode, and nucleosynthetic yields. The most efficient of the standard wind rates frequently used in the literature seems to favour agreement between our yield results and observational data. Regardless of the wind prescription, all our models become N-enhanced EMP stars.
We present the evolutionary models of metal-free stars in the mass range from 0.8 to 1.2 Msun with up-to-date input physics. The evolution is followed to the onset of hydrogen mixing into a convection, driven by the helium flash at red giant or asymptotic giant branch phase. The models of mass M >= 0.9 Msun undergo the central hydrogen flash, triggered by the carbon production due to the 3-alpha reactions. We find that the border of the off-center and central ignition of helium core flash falls between 1.1 and 1.2 Msun; the models of mass M <= 1.1 Msun experience the hydrogen mixing at the tip of red giant branch while the models of M = 1.2 Msun during the helium shell flashes on the asymptotic giant branch. The equation of state for the Coulomb liquid region, where electron conduction and radiation compete, is shown to be important since it affects the thermal state in the helium core and influences the red giant branch evolution. It is also found that the non-resonant term of 3-alpha reactios plays an important role, although it has negligible effect in the evolution of stars of younger populations. We compare our models with the computations by several other sets of authors, to confirm the good agreement except for one study which finds the helium ignition much closer to the center with consequences important for subsequent evolution.
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