arXiv subjects

E. Caux

Publications and source records attributed to E. Caux.

90 records · Page 5Linked to original sources

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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arXiv2006

Extreme Deuteration and Hot Corinos: the Earliest Chemical Signatures of Low-Mass Star Formation

Low-mass protostars form from condensations inside molecular clouds when gravity overwhelms thermal and magnetic supporting forces. The first phases of the formation of a solar-type star are characterized by dramatic changes not only in the physical structure but also in the chemical composition. Since PPIV (e.g., Langer et al.), exciting new developments have occurred in our understanding of the processes driving this chemical evolution. These developments include two new discoveries : 1) extremely enhanced molecular deuteration, which is caused by the freeze-out of heavy-element-bearing molecules onto grain mantles during the Prestellar Core and Class 0 source phases; and 2) hot corinos, which are warm and dense regions at the center of Class 0 source envelopes and which are characterized by a multitude of complex organic molecules. In this chapter we will review these two new topics, and will show how they contribute to our understanding of the first phases of solar-type stars.

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arXiv2005

Discovery of deuterated water in a young proto-planetary disk

We report the first detection of the ground transition of the deuterated water at 464 GHz in the young proto-planetary disk surrounding the solar type protostar DM Tau. The line is observed in absorption against the continuum from the cold dust in the disk midplane, with a line to continuum ratio close to unity. The observation implies that deuterated gaseous water is present, with a relatively large abundance ($\sim 3\times10^{-9}$), in the outer disk above the midplane, where the density is, within a factor ten, $\sim 10^6$ cm$^{-3}$ and the temperature is lower than about 25 K. In these conditions, the H$_2$O condensation timescale is much smaller than the DM Tau disk age, and, therefore, water should be fully frozen onto the grain mantles. We suggest that UV photons and/or X-rays sublimate part of the mantles re-injecting the ices into the gas phase. Even though there is currently no measurement of H$_2$O, we provide arguments that the HDO/H$_2$O ratio should be about 0.01 or larger, which would be hundreds of times larger than the values measured in Solar System objects. This suggests the need of strong caution in comparing and linking the HDO/H$_2$O in Solar System and star forming environments.

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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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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

HDO abundance in the envelope of the solar-type protostar IRAS16293-2422

We present IRAM 30m and JCMT observations of HDO lines towards the solar-type protostar IRAS 16293-2422. Five HDO transitions have been detected on-source, and two were unfruitfully searched for towards a bright spot of the outflow of IRAS 16293-2422. We interpret the data by means of the Ceccarelli, Hollenbach and Tielens (1996) model, and derive the HDO abundance in the warm inner and cold outer parts of the envelope. The emission is well explained by a jump model, with an inner abundance of 1e-7 and an outer abundance lower than 1e-9 (3 sigma). This result is in favor of HDO enhancement due to ice evaporation from the grains in theinner envelope. The deuteration ratio HDO/H2O is found to be f_in=3% and f_out < 0.2% (3 sigma) in the inner and outer envelope respectively and therefore, the fractionation also undergoes a jump in the inner part of the envelope. These results are consistent with the formation of water in the gas phase during the cold prestellar core phase and storage of the molecules on the grains, but do not explain why observations of H2O ices consistently derive a H2O ice abundance of several 1e-5 to 1e-4, some two orders of magnitude larger than the gas phase abundance of water in the hot core around IRAS 16293-2422.

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arXiv2003

First detection of triply-deuterated methanol

We report the first detection of triply-deuterated methanol, with 12 observed transitions, towards the low-mass protostar IRAS 16293-2422, as well as multifrequency observations of 13CH3OH, used to derive the column density of the main isotopomer CH3OH. The derived fractionation ratio [CD3OH]/[CH3OH] averaged on a 10'' beam is 1.4%. Together with previous CH2DOH and CHD2OH observations, the present CD3OH observations are consistent with a formation of methanol on grain surfaces, if the atomic D/H ratio is 0.1 to 0.3 in the accreting gas. Such a high atomic ratio can be reached in the frame of gas-phase chemical models including all deuterated isotopomers of H3+.

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arXiv2003

Sulphur-bearing species in the star forming region L1689N

We report observations of the expected main S-bearing species (SO, SO2 and H2S) in the low-mass star forming region L1689N. We obtained large scale (~300''x200'') maps of several transitions from these molecules with the goal to study the sulphur chemistry, i.e. how the relative abundances change in the different physical conditions found in L1689N. We identified eight interesting regions, where we carried out a quantitative comparative study: the molecular cloud (as reference position), five shocked regions caused by the interaction of the molecular outflows with the cloud, and the two protostars IRAS16293-2422 and 16293E. In the cloud we carefully computed the gas temperature and density by means of a non-LTE LVG code, while in other regions we used previous results. We hence derived the column density of SO, SO2 and H2S, together with SiO and H2CO - which were observed previously - and their relevant abundance ratios. We find that SiO is the molecule that shows the largest abundance variations in the shocked regions, whereas S-bearing molecules show more moderate variations. Remarkably, the region of the brightest SiO emission in L1689N is undetected in SO2, H2S and H2CO and only marginally detected in SO. In the other weaker SiO shocks, SO2 is enhanced with respect to SO. We propose a schema in which the different molecular ratios correspond to different ages of the shocks. Finally, we find that SO, SO2 and H2S have significant abundance jumps in the inner hot core of IRAS16293-2422 and discuss the implications of the measured abundances.

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arXiv2003

Search for solid HDO in low-mass protostars

We present ground-based 2.1 to 4.2 microns observations of four low-mass protostars. We searched for the 4.1 microns OD stretch band, characteristic of solid HDO in grain mantles. We did not detect solid HDO in any of the four sources, but we derive 3-sigma upper limits from 0.5% to 2% for the HDO/H2O ratio depending on the source. These ratios provide strong constraints to solid-state deuteration models when compared to deuterium fractionation values observed in the gas phase. We discuss various scenarios that could lead to such a low water deuteration compared to the high formaldehyde and methanol deuteration observed in the gas phase.

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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

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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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arXiv2002

Detection of doubly-deuterated methanol in the solar-type protostar IRAS16293-2422

We report the first detection of doubly-deuterated methanol (CHD2OH), as well as firm detections of the two singly-deuterated isotopomers of methanol (CH2DOH and CH3OD), towards the solar-type protostar IRAS16293-2422. From the present multifrequency observations, we derive the following abundance ratios: [CHD2OH]/[CH3OH] = 0.2 +/- 0.1, [CH2DOH]/[CH3OH] = 0.9 +/- 0.3, [CH3OD]/[CH3OH] = 0.04 +/- 0.02. The total abundance of the deuterated forms of methanol is greater than that of its normal hydrogenated counterpart in the circumstellar material of IRAS16293-2422, a circumstance not previously encountered. Formaldehyde, which is thought to be the chemical precursor of methanol, possesses a much lower fraction of deuterated isotopomers (~ 20%) with respect to the main isotopic form in IRAS16293-2422. The observed fractionation of methanol and formaldehyde provides a severe challenge to both gas-phase and grain-surface models of deuteration. Two examples of the latter model are roughly in agreement with our observations of CHD2OH and CH2DOH if the accreting gas has a large (0.2-0.3) atomic D/H ratio. However, no gas-phase model predicts such a high atomic D/H ratio, and hence some key ingredient seems to be missing.

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arXiv2002

Detection of HD in emission towards Sagittarius B2

The giant molecular cloud Sagittarius B2, located near the Galactic Centre, has been observed in the far-infrared by the ISO Long Wavelength Spectrometer. Wavelengths in the range 47-196 microns were covered with the high resolution Fabry-Perot spectrometer, giving a spectral resolution of 30-40 km/s. The J=1-0 and J=2-1 rotational transitions of HD fall within this range at 112 microns and 56 microns. A probable detection was made of the ground state J=1-0 line in emission but the J=2-1 line was not detected above the noise. This allowed us to calculate an upper limit on the temperature in the emitting region of approximately 80 K and a value for the deuterium abundance in the Sgr B2 envelope of D/H=(0.2-11)x10^-6.

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arXiv2000

The ISO-LWS map of the Serpens cloud core: I. The SEDs of the IR/SMM sources

ISO-LWS mapping observations of the Serpens molecular cloud core are presented. The spectral range is 50 - 200 micron and the map size is 8' X 8'. These observations suffer from severe source confusion at FIR wavelengths and we employ a Maximum Likelihood Method for the spectro-spatial deconvolution. The strong and fairly isolated source SMM1 FIRS1 presented a test case, whose modelled spectral energy distribution (SED), within observational errors, is identical to the observed one. The model results for the other infrared and submillimetre sources are therefore likely to represent their correct SEDs. Simulations demonstrating the reliability and potential of the developed method support this view. For the majority of sources the peak of the SEDs is found within the spectral range of the LWS and derived temperatures are generally higher (>30K) than have been found by earlier deconvolution attempts using IRAS data. SMM sizes are found to be only a few arcsec in diameter. In addition, the SMMs are generally optically thick even at LWS wavelengths. Self-consistent radiative transfer calculations indicates that the SMMs are optically thick out to even longer wavelengths. Models were calculated for five sources, for which sufficient data were available, viz. SMM 1, 2, 3, 4 and 9. These models are optically thick out to millimetre wavelengths (wavelength of unit optical depth 900 to 1400 micron). Envelope masses for these SMMs are in the range 2-6 Msun, which is of course considerably more massive than estimates based on the optically thin assumption. The luminosities are in the range 10-70 Lsun, suggesting the formation of low-mass to intermediate mass stars, so that the existence of such massive envelopes argues for extreme youth of the SMMs in the Serpens cloud core.

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arXiv2000

ISO Spectroscopy of the HH 7-11 Flow and its red-shifted Counterpart

We have used the two spectrometers on the Infrared Space Observatory to observe the HH 7-11 flow, its red-shifted counterpart, and the candidate exciting source SVS 13, in the star formation region NGC 1333. We detect atomic ([OI]63um and 145um, [SiII]34.8um, [CII]158um) and molecular (H_2, CO, H_2O) lines at various positions along the bipolar flow. Most of the observed lines can be explained in terms of shock-excited emission. In particular, our analysis shows that dissociative (J-type) and non-dissociative (C-type) shocks are simultaneously present everywhere along both lobes of the flow. We confirm the low-excitation nature of the Herbig-Haro nebulosities, with shock velocities <40-50 km/s. Toward both lobes of the outflow we find pre-shock densities of n_0~10^4 cm-3 for both the J and C components, implying B_0~100uG. In the central region of the flow, close to the exciting source, the pre-shock density deduced for the C-shock component is n_0~10^5 cm-3, suggesting a magnetic field ~3 times stronger. We propose that the deficiency of gas-phase water in the post C-shock regions is due to freezing onto warm grains processed through the J-shock front and traveling along the magnetic field lines. The total observed cooling from the dissociative shock components is consistent with the power lost by a slow molecular outflow accelerated by a fast neutral HI wind. Finally, the skin of the cloud seen in projection toward the flow appears to be weakly photo-ionised by BD +30 549, the dominant illuminating source of the NGC 1333 reflection nebula.

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arXiv1999

Shock excited far-infrared molecular emission around T Tau

The first complete far-infrared spectrum of T Tau has been obtained with the LWS spectrometer on-board the Infrared Space Observatory, which detected strong emission from high-J (J=14-25) CO, para- and ortho-H2O and OH transitions over the wavelength range from 40 to 190 micron. Most of the observed molecular emission can be explained by a single emission region at T~300-900 K and n(H_2)~10^(5-6) cm^(-3),with a diameter of about 2-3 arcsec. This corresponds to a very compact region of 300-400 AU at the distance of 140 pc. An higher temperature component seems to be needed to explain the highest excitation CO and water lines. We derive a water abundance of 1-7x10^(-5) and an OH abundance of ~3x10^(-5) with respect to molecular hydrogen, implying water and OH enhancements by more than a factor of 10 with respect to the expected ambient gas abundance. The observed cooling in the various species amounts to 0.04 L(sun),comparable to the mechanical luminosity of the outflow, indicating that the stellar winds could be responsible of the line excitation through shocks. In order to explain the observed molecular cooling in T Tau in terms of C-type shock models, we hypothesise that the strong far-ultraviolet radiation field photodissociates water in favour of OH. This would explain the large overabundance of OH observed. The estimated relatively high density and compactness of the observed emission suggest that it originates from the shocks taking place at the base of the molecular outflow emission, in the region where the action of the stellar winds from the two stars of the binary system is important.

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