S. Maret — Page 4 | Searcharxiv

arXiv2006

Testing grain surface chemistry : a survey of deuterated formaldehyde and methanol in low-mass Class 0 protostars

Context : Despite the low cosmic abundance of deuterium (D/H ~ 1e-5), large degrees of deuterium fractionation in molecules are observed in star forming regions with enhancements that can reach 13 orders of magnitude, which current models have difficulties to account for. Aims : Multi-isotopologue observations are a very powerful constraint for chemical models. The aim of our observations is to understand the processes forming the observed large abundances of methanol and formaldehyde in low-mass protostellar envelopes (gas-phase processes ? chemistry on the grain surfaces ?) and better constrain the chemical models. Methods : Using the IRAM 30m single-dish telescope, we observed deuterated formaldehyde (HDCO and D2CO) and methanol (CH2DOH, CH3OD, and CHD2OH) towards a sample of seven low-mass class 0 protostars. Using population diagrams, we then derive the fractionation ratios of these species (abundance ratio between the deuterated molecule and its main isotopologue) and compare them to the predictions of grain chemistry models. Results : These protostars show a similar level of deuteration as in IRAS16293-2422, where doubly-deuterated methanol -- and even triply-deuterated methanol -- were first detected. Our observations point to the formation of methanol on the grain surfaces, while formaldehyde formation cannot be fully pined down. While none of the scenarii can be excluded (gas-phase or grain chemistry formation), they both seem to require abstraction reactions to reproduce the observed fractionations.

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arXiv2005

Methanol emission from low mass protostars

We present observations of methanol lines in a sample of Class 0 low mass protostars. Using a 1-D radiative transfer model, we derive the abundances in the envelopes. In two sources of the sample, the observations can only be reproduced by the model if the methanol abundance is enhanced by about two order of magnitude in the inner hot region of the envelope. Two other sources show similar jumps, although at a lower confidence level. The observations for the other three sources are well reproduced with a constant abundance, but the presence of a jump cannot be ruled out. The observed methanol abundances in the warm gas around low mass protostars are orders of magnitude higher than gas phase chemistry models predict. Hence, in agreement with other evidences, this suggest that the high methanol abundance reflects recent evaporation of ices due to the heating by the newly formed star. The observed abundance ratios of CH3 OH, H2 CO, and CO are in good agreement with grain surface chemistry models. However, the absolute abundances are more difficult to reproduce and may point towards the presence of multiple ice components in these regions.

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arXiv2005

Theoretical HDO emission from low-mass protostellar envelopes

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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arXiv2004

A submillimeter line survey of low-mass protostars: prelude to ALMA and Herschel

The results from a single-dish molecular line survey of a set of 18 deeply embedded young stellar objects are summarized. More than 40 lines from 16 different species were observed with the JCMT, Onsala, IRAM 30m and SEST telescopes. The multi-transition data are analyzed using a temperature and density structure derived from models of the dust continuum emission. For the outer envelope (>300 AU), the data indicate a `drop' abundance profile for many species, with normal abundances in the outer- and innermost regions and highly depleted abundances in an intermediate zone. This zone is bounded at the outer than the lifetime of the core, and at the inner edge by the evaporation temperature of the species involved. In the innermost envelope (<300 AU), all ices evaporate resulting in jumps in the abundances of complex organic molecules such as CH3OH. A key project for Herschel will be to survey gas-phase water in these objects, whose abundance shows extreme variations with temperature. ALMA wil be able to directly image the chemical variations throughout the envelope and zoom in on the inner hot core and protoplanetary disks on scales on tens of AU.

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arXiv2004

Methanol abundance in low mass protostars

Methanol lines observations of a sample of low mass Class 0 protostars are presented. Using a 1D radiative transfer model, I show that several protostars have large abundance jumps in the inner hot and dense region of envelopes, probably because of thermal grain mantle evaporation. These abundances are compared with a grain surface chemistry model.

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arXiv2003

The far infrared CO line emission of Orion BN/KL

We present observations towards one of the closest regions of high mass star formation, Orion BN/KL, performed at both low resolution mode (grating mode) and high resolution mode (Fabry-Perot) with the Long Wavelength Spectrometer on board the Infrared Space Observatory. We detected the CO rotational lines from Jup = 15 to Jup = 45. While the lines with Jup<= 32 are spectrally unresolved, the higher lying lines show a broadened profile. Finally, we detected two 13CO lines, namely at Jup = 18 and 24, from which we could derive the opacities of the relative 12CO lines. The LVG analysis of the observed line spectrum allows to distinguish three main physical components with different temperatures, densities and column densities: 1) lines with Jup< 20 originate mainly in the diffuse photodissociation region surrounding the source; 2) lines with Jup between 20 and 30 originate in the high velocity outflow (plateau) emanating from IrC2; 3) lines with Jup > 32 originate in the hot and dense gas of the shocked component of the outflow. We discuss how future observations with HIFI, onboard the Far Infrared Space Telescope (FIRST) will allow to spectrally and spatially disentangle the three components, and, consequently, characterise more precisely the Orion BN/KL star forming region.

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arXiv2003

Water emission in NGC1333-IRAS4: Probing its surrounding envelope

Sun like stars are born from the collapse of fragment of molecular clouds. During the first embedded phase (the so called class 0/1), the protostar is surrounded by a collapsing envelope, whose physical and chemical structure would set up the initial conditions of the proto-stellar disk which may eventually form planets. In this contribution, we show how spectroscopic far infrared to millimeter observations can be a powerful diagnostic to derive the chemical and physical structure of the innermost regions of envelopes collapsing around solar type protostars, where disk are believe to form. Our claim is based on the simultaneous modeling of the dynamics, chemistry and radiative transfer in a protostellar envelope. Here we present a study of a class 0 protostar NGC1333-IRAS4, obtained using ISO-LWS observations of water far-infrared lines. We show the derived physical and chemical structure of the protostar, namely the density and temperature profiles, as well as the water abundance across the envelope. Besides, the mass of the central protostar and its accretion rate can be constrained by these observations and modeling, yielding an alternative method to measure these two poorly know parameters.

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arXiv2003

Physical and Chemical Structure of the Solar Type Protostar NGC1333-IRAS4

Sun like stars are born from the collapse of fragment of molecular clouds. During the first embedded phase (the so called class 0/1), the protostar is surrounded by a collapsing envelope, whose physical and chemical structure would set up the initial conditions of the proto-stellar disk which may eventually form planets. In this contribution, we show how spectroscopic far infrared to millimeter observations can be a powerful diagnostic to derive the chemical and physical structure of the innermost regions of envelopes collapsing around solar type protostars, where disk are believe to form. Our claim is based on the simultaneous modeling of the dynamics, chemistry and radiative transfer in a protostellar envelope. Here we present a study of a class 0 protostar NGC1333-IRAS4, obtained using ISO-LWS observations of water far-infrared lines. We show the derived physical and chemical structure of the protostar, namely the density and temperature profiles, as well as the water abundance across the envelope. Besides, the mass of the central protostar and its accretion rate can be constrained by these observations and modeling, yielding an alternative method to measure these two poorly know parameters.

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arXiv2002

Water emission in NGC1333-IRAS4: The physical structure of the envelope

We report ISO-LWS far infrared observations of CO, water and oxygen lines towards the protobinary system IRAS4 in the NGC1333 cloud. We detected several water, OH, CO rotational lines, and two [OI] and [CII] fine structure lines. Given the relatively poor spectral and spatial resolution of these observations, assessing the origin of the observed emission is not straightforward. In this paper, we focus on the water line emission and explore the hypothesis that it originates in the envelopes that surround the two protostars, IRAS4 A and B, thanks to an accurate model. The model reproduces quite well the observed water line fluxes, predicting a density profile, mass accretion rate, central mass, and water abundance profile in agreement with previous works. We hence conclude that the emission from the envelopes is a viable explanation for the observed water emission, although we cannot totally rule out the alternative that the observed water emission originates in the outflow.

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