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Register a new userTheoretical HDO emission from low-mass protostellar envelopes
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We present theoretical predictions of the rotational line emission of deuterated water in low-mass protostar collapsing envelopes. The model accounts for the density and temperature structure of the envelope, according the inside-out collapse framework. The deuterated water abundance profile is approximated by a step function, with a low value in the cold outer envelope and a higher value in the inner envelope where the grain mantles evaporate. The two abundances are the two main parameters of the modeling, along with the temperature at which the mantles evaporate. We report line flux predictions for a 30 and 5 L$_odot$ source luminosity respectively. We show that ground based observations are capable to constrain the three parameters of the model in the case of bright low-mass protostars (L$>$10 L$_{odot}$), and that no space based observations, like for example HSO observations, are required in this case. On the contrary, we show that the study of low-luminosity sources (L$<$10 L$_{odot}$), assuming the same HDO abundance profile, requires too much integration time to be carried out either with available ground-based telescopes or with the HIFI instrument on board HSO. For these sources, only the large interferometer ALMA will allow to constrain the HDO abundance.
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Within low-mass star formation, water vapor plays a key role in the chemistry and energy balance of the circumstellar material. The Herschel Space Observatory will open up the possibility to observe water lines originating from a wide range of excitation energies.Our aim is to simulate the emission of rotational water lines from envelopes characteristic of embedded low-mass protostars. A large number of parameters that influence the water line emission are explored: luminosity, density,density slope and water abundances.Both dust and water emission are modelled using full radiative transfer in spherical symmetry. The temperature profile is calculated for a given density profile. The H2O level populations and emission profiles are in turn computed with a non-LTE line code. The results are analyzed to determine the diagnostic value of different lines, and are compared with existing observations. Lines can be categorized in: (i) optically thick lines, including ground-state lines, mostly sensitive to the cold outer part; (ii) highly excited (E_u>200-250 K) optically thin lines sensitive to the abundance in the hot inner part; and (iii) lines which vary from optically thick to thin depending on the abundances. Dust influences the emission of water significantly by becoming optically thick at the higher frequencies, and by pumping optically thin lines. A good physical model of a source, including a correct treatment of dust, is a prerequisite to infer the water abundance structure and possible jumps at the evaporation temperature from observations. The inner warm (T>100 K) envelope can be probed byhighly-excited lines, while a combination of excited and spectrally resolved ground state lines probes the outer envelope. Observations of H218O lines, although weak, provide even stronger constraints on abundances.
Recent interferometer observations have found that the D2O/HDO abundance ratio is higher than that of HDO/H2O by about one order of magnitude in the vicinity of low-mass protostar NGC 1333-IRAS 2A, where water ice has sublimated. Previous laboratory and theoretical studies show that the D2O/HDO ice ratio should be lower than the HDO/H2O ice ratio, if HDO and D2O ices are formed simultaneously with H2O ice. In this work, we propose that the observed feature, D2O/HDO > HDO/H2O, is a natural consequence of chemical evolution in the early cold stages of low-mass star formation: 1) majority of oxygen is locked up in water ice and other molecules in molecular clouds, where water deuteration is not efficient, and 2) water ice formation continues with much reduced efficiency in cold prestellar/protostellar cores, where deuteration processes are highly enhanced due to the drop of the ortho-para ratio of H2, the weaker UV radiation field, etc. Using a simple analytical model and gas-ice astrochemical simulations tracing the evolution from the formation of molecular clouds to protostellar cores, we show that the proposed scenario can quantitatively explain the observed HDO/H2O and D2O/HDO ratios. We also find that the majority of HDO and D2O ices are likely formed in cold prestellar/protostellar cores rather than in molecular clouds, where the majority of H2O ice is formed. This work demonstrates the power of the combination of the HDO/H2O and D2O/HDO ratios as a tool to reveal the past history of water ice formation in the early cold stages of star formation and when the enrichment of deuterium in the bulk of water occurred. Further observations are needed to explore if the relation, D2O/HDO > HDO/H2O, is common in low-mass protostellar sources.
Recent water line observations toward several low-mass protostars suggest low water gas fractional abundances in the inner warm envelopes. Water destruction by X-rays has been proposed to influence the water abundances in these regions, but the detailed chemistry, including the nature of alternative oxygen carriers, is not yet understood. In this study, we aim to understand the impact of X-rays on the composition of low-mass protostellar envelopes, focusing specifically on water and related oxygen bearing species. We compute the chemical composition of two low-mass protostellar envelopes using a 1D gas-grain chemical reaction network, under various X-ray field strengths. According to our calculations, outside the water snowline, the water gas abundance increases with $L_{mathrm{X}}$. Inside the water snowline, water maintains a high abundance of $sim 10^{-4}$ for small $L_{mathrm{X}}$, with water and CO being the dominant oxygen carriers. For large $L_{mathrm{X}}$, the water gas abundances significantly decrease just inside the water snowline (down to $sim10^{-8}-10^{-7}$) and in the innermost regions ($sim10^{-6}$). For these cases, the O$_{2}$ and O gas abundances reach $sim 10^{-4}$ within the water snowline, and they become the dominant oxygen carriers. The HCO$^{+}$ and CH$_{3}$OH abundances, which have been used as tracers of the water snowline, significantly increase/decrease within the water snowline, respectively, as the X-ray fluxes become larger. The abundances of some other dominant molecules, such as CO$_{2}$, OH, CH$_{4}$, HCN, and NH$_{3}$, are also affected by strong X-ray fields, especially within their own snowlines. These X-ray effects are larger in lower density envelope models. Future observations of water and related molecules (using e.g., ALMA and ngVLA) will access the regions around protostars where such X-ray induced chemistry is effective.
We have made mapping observations of L1551 IRS 5, L1551NE, L723, and L43 and single-point observations of IRAS 16293-2422 in the submillimeter CS (J = 7-6) and HCN (J = 4-3) lines with ASTE. Including our previous ASTE observations of L483 and B335, we found a clear linear correlation between the source bolometric luminosities and the total integrated intensities of the submillimeter lines (I_CS ~L_bol^0.92). The combined ASTE + SMA CS (7-6) image of L1551 IRS 5 exhibits an extended (~2000 AU) component tracing the associated reflection nebula at the west and southwest, as well as a compact (< 500 AU) component centered on the protostellar position. The emission peaks of the CS and HCN emissions in L1551 NE are not located at the protostellar position but offset (~1400 AU) toward the associated reflection nebula at the west. With the statistical analyses, we confirmed the opposite velocity gradients of the CS (7-6) emission to those of the millimeter lines along the outflow direction, which we reported in our early paper. The magnitudes of the submillimeter velocity gradients are estimated to be (9.7pm1.7) times 10-3 km s-1 arcsec-1 in L1551 IRS 5 and (7.6pm2.4) times 10-3 km s-1 arcsec-1 in L483. We suggest that the skewed submillimeter molecular emissions toward the associated reflection nebulae at a few thousands AU scale trace the warm (> 40 K) walls of the envelope cavities, excavated by the associated outflows and irradiated by the central protostars directly. The opposite velocity gradients along the outflow direction likely reflect the dispersing gas motion at the wall of the cavity in the envelopes perpendicular to the outflow.
How and when in the star formation sequence do dust grains start to grow into pebbles is a cornerstone question to both star and planet formation. We compute the polarized radiative transfer from a model solar-type protostellar core, using the POLARIS code, aligning the dust grains with the local magnetic field, following the radiative torques (RATs) theory. We test the dependency of the resulting dust polarized emission with the maximum grain size of the dust size distribution at the envelope scale, from amax = 1 micron to 50 micron. Our work shows that, in the framework of RAT alignment, large dust grains are required to produce polarized dust emission at levels similar to those currently observed in solar-type protostellar envelopes at millimeter wavelengths. Considering the current theoretical dificulties to align a large fraction of small ISM-like grains in the conditions typical of protostellar envelopes, our results suggest that grain growth (typically > 10 micron) might have already significantly progressed at scales 100-1000 au in the youngest objects, observed less than 10^5 years after the onset of collapse. Observations of dust polarized emission might open a new avenue to explore dust pristine properties and describe, for example, the initial conditions for the formation of planetesimals.
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