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Register a new userTesting grain surface chemistry : a survey of deuterated formaldehyde and methanol in low-mass Class 0 protostars
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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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Methanol is a key species in astrochemistry since it is the most abundant organic molecule in the ISM and is thought to be the mother molecule of many complex organic species. Estimating the deuteration of methanol around young protostars is of crucial importance because it highly depends on its formation mechanisms and the physical conditions during its moment of formation. We analyse dozens of transitions from deuterated methanol isotopologues coming from various existing observational datasets from the IRAM-PdBI and ALMA sub-mm interferometers to estimate the methanol deuteration surrounding three low-mass protostars on Solar System scales. A population diagram analysis allows us to derive a [CH$_2$DOH]/[CH$_3$OH] abundance ratio of 3-6 % and a [CH$_3$OD]/[CH$_3$OH] ratio of 0.4-1.6 % in the warm inner protostellar regions. These values are ten times lower than those derived with previous single-dish observations towards these sources but they are 10-100 times higher than the methanol deuteration measured in massive hot cores. Dust temperature maps obtained from Herschel and Planck observations show that massive hot cores are located in warmer molecular clouds than low-mass sources, with temperature differences of $sim$10 K. Comparison with the predictions of the gas-grain astrochemical model GRAINOBLE shows that such a temperature difference is sufficient to explain the different deuteration observed in low- to high-mass sources, suggesting that the physical conditions of the molecular cloud at the origin of the protostars mostly govern the present observed deuteration of methanol. The methanol deuteration measured in this work is higher by a factor of 5 than the upper limit in methanol deuteration estimated in comet Hale-Bopp, implying that an important reprocessing of the organic material would have occurred in the solar nebula during the formation of the Solar System.
Deuterated methanol is one of the most robust windows astrochemists have on the individual chemical reactions forming deuterium-bearing molecules and the physicochemical history of the regions where they reside. The first-time detection of mono- and di-deuterated methanol in a cometary coma is presented for comet 67P/Churyumov-Gerasimenko using Rosetta-ROSINA data. D-methanol (CH3OD and CH2DOH combined) and D2-methanol (CH2DOD and CHD2OH combined) have an abundance of 5.5+/-0.46 and 0.00069+/-0.00014 per cent relative to normal methanol. The data span a methanol deuteration fraction (D/H ratio) in the 0.71-6.6 per cent range, accounting for statistical corrections for the location of D in the molecule and including statistical error propagation in the ROSINA measurements. It is argued that cometary CH2DOH forms from CO hydrogenation to CH3OH and subsequent H-D substitution reactions in CH3-R. CHD2OH is likely produced from deuterated formaldehyde. Meanwhile, CH3OD and CH2DOD, could form via H-D exchange reactions in OH-R in the presence of deuterated water ice. Methanol formation and deuteration is argued to occur at the same epoch as D2O formation from HDO, with formation of mono-deuterated water, hydrogen sulfide, and ammonia occurring prior to that. The cometary D-methanol/methanol ratio is demonstrated to agree most closely with that in prestellar cores and low-mass protostellar regions. The results suggest that cometary methanol stems from the innate cold (10-20 K) prestellar core that birthed our Solar System. Cometary volatiles individually reflect the evolutionary phases of star formation from cloud to core to protostar.
With the Stratospheric Observatory for Infrared Astronomy (SOFIA) routinely operating science flights, we demonstrate that observations with the Faint Object infraRed CAmera for the SOFIA Telescope (FORCAST) can provide reliable estimates of the internal luminosities, $L_{rm int}$, of protostars. We have developed a technique to estimate $L_{rm int}$ using a pair of FORCAST filters: one short-wavelength filter centered within 19.7-25.3 $mu$m, and one long-wavelength filter within 31.5-37.1 $mu$m. These $L_{rm int}$ estimates are reliable to within 30-40% for 67% of protostars and to within a factor of 2.3-2.6 for 99% of protostars. The filter pair comprised of F25.3$mu$m and F37.1$mu$m achieves the best sensitivity and most constrained results. We evaluate several assumptions that could lead to systematic uncertainties. The OH5 dust opacity matches observational constraints for protostellar environments best, though not perfectly; we find that any improved dust model will have a small impact of 5-10% on the $L_{rm int}$ estimates. For protostellar envelopes, the TSC84 model yields masses that are twice those of the Ulrich model, but we conclude this mass difference does not significantly impact results at the mid-infrared wavelengths probed by FORCAST. Thus, FORCAST is a powerful instrument for luminosity studies targeting newly discovered protostars or suspected protostars lacking detections longward of 24 $mu$m. Furthermore, with its dynamic range and greater angular resolution, FORCAST may be used to characterize protostars that were either saturated or merged with other sources in previous surveys using the Spitzer Space Telescope or Herschel Space Observatory.
Four Class I maser sources were detected at 44, 84, and 95 GHz toward chemically rich outflows in the regions of low-mass star formation NGC 1333I4A, NGC 1333I2A, HH25, and L1157. One more maser was found at 36 GHz toward a similar outflow, NGC 2023. Flux densities of the newly detected masers are no more than 18 Jy, being much lower than those of strong masers in regions of high-mass star formation. The brightness temperatures of the strongest peaks in NGC 1333I4A, HH25, and L1157 at 44 GHz are higher than 2000 K, whereas that of the peak in NGC 1333I2A is only 176 K. However, rotational diagram analysis showed that the latter source is also a maser. The main properties of the newly detected masers are similar to those of Class I methanol masers in regions of massive star formation. The former masers are likely to be an extension of the latter maser population toward low luminosities of both the masers and the corresponding YSOs.
The IRDC SDC335.579-0.292 (SDC335) is a massive star-forming cloud found to be globally collapsing towards one of the most massive star forming cores in the Galaxy. SDC335 hosts three high-mass protostellar objects at early stages of their evolution and archival ALMA Cycle 0 data indicate the presence of at least one molecular outflow in the region. Observations of molecular outflows from massive protostellar objects allow us to estimate the accretion rates of the protostars as well as to assess the disruptive impact that stars have on their natal clouds. The aim of this work is to identify and analyse the properties of the protostellar-driven molecular outflows within SDC335 and use these outflows to help refine the properties of the protostars. We imaged the molecular outflows in SDC335 using new data from the ATCA of SiO and Class I CH$_3$OH maser emission (~3 arcsec) alongside observations of four CO transitions made with APEX and archival ALMA CO, $^{13}$CO (~1 arcsec), and HNC data. We introduced a generalised argument to constrain outflow inclination angles based on observed outflow properties. We used the properties of each outflow to infer the accretion rates on the protostellar sources driving them and to deduce the evolutionary characteristics of the sources. We identify three molecular outflows in SDC335, one associated with each of the known compact HII regions. The outflow properties show that the SDC335 protostars are in the early stages (Class 0) of their evolution, with the potential to form stars in excess of 50 M$_{odot}$. The measured total accretion rate onto the protostars is $1.4(pm 0.1) times 10^{-3}$M$_{odot}$ yr$^{-1}$, comparable to the total mass infall rate toward the cloud centre on parsec scales of 2.5$(pm 1.0) times 10^{-3}$M$_{odot}$ yr$^{-1}$, suggesting a near-continuous flow of material from cloud to core scales. [abridged].
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