Disk masses in the Orion Molecular Cloud-2: distinguishing time and environment

The mass evolution of protoplanetary disks is driven by both internal processes and external factors, such as photoevaporation. Disentangling these two effects, however, has remained difficult. We measure the dust masses of a sample of 132 disks in the Orion Molecular Cloud (OMC)-2 region, and compare them to (i) externally photoevaporated disks in the Trapezium cluster, and (ii) disks in nearby low-mass star forming regions (SFRs). This allows us to test if initial disk properties are the same in high- and low-mass SFRs, and enables a direct measurement of the effect of external photoevaporation on disks. A ~ $20' \times 4'$ mosaic of 3 mm continuum observations from the Atacama Large Millimeter/submillimeter Array (ALMA) was used to measure the fluxes of 132 disks and 35 protostars >0.5 pc away from the Trapezium. We identify and characterize a sample of 34 point sources not included in the Spitzer catalog on which the sample is based. Of the disks, 37 (28%) are detected, with masses ranging from 7-270 M_e. The detection rate for protostars is higher at 69%. Disks near the Trapezium are found to be less massive by a factor $0.18^{+0.18}_{-0.11}$, implying a mass loss rate of $8\times10^{-8}$ M_sun/yr. Our observations allow us to distinguish the impact of time and environment on disk evolution in a single SFR. The disk mass distribution in OMC-2 is statistically indistinguishable from that in nearby low-mass SFRs, like Lupus and Taurus. We conclude that age is the main factor determining the evolution of these disks. This result is robust with respect to assumptions of dust temperature, sample incompleteness and biases. The difference between the OMC-2 and Trapezium cluster samples is consistent with mass loss driven by far-ultraviolet radiation near the Trapezium. Together, this implies that in isolation, disk formation and evolution proceed similarly, regardless of cloud mass.Comment: Accepted for publication in A&A. 16 pages, 6 figure

Search for methylamine in high mass hot cores

We aim to detect methylamine, CH$_{3}$NH$_{2}$, in a variety of hot cores and use it as a test for the importance of photon-induced chemistry in ice mantles and mobility of radicals. Specifically, CH$_3$NH$_2$ cannot be formed from atom addition to CO whereas other NH$_2$-containing molecules such as formamide, NH$_2$CHO, can. Submillimeter spectra of several massive hot core regions were taken with the James Clerk Maxwell Telescope. Abundances are determined with the rotational diagram method where possible. Methylamine is not detected, giving upper limit column densities between 1.9 $-$ 6.4 $\times$ 10$^{16}$ cm$^{-2}$ for source sizes corresponding to the 100 K envelope radius. Combined with previously obtained JCMT data analyzed in the same way, abundance ratios of CH$_{3}$NH$_{2}$, NH$_{2}$CHO and CH$_{3}$CN with respect to each other and to CH$_{3}$OH are determined. These ratios are compared with Sagittarius B2 observations, where all species are detected, and to hot core models. The observed ratios suggest that both methylamine and formamide are overproduced by up to an order of magnitude in hot core models. Acetonitrile is however underproduced. The proposed chemical schemes leading to these molecules are discussed and reactions that need further laboratory studies are identified. The upper limits obtained in this paper can be used to guide future observations, especially with ALMA.Comment: 14 pages, 5 figures, accepted for publication in A&

Photodissociation and photoionisation of atoms and molecules of astrophysical interest

A new collection of photodissociation and photoionisation cross sections for 102 atoms and molecules of astrochemical interest has been assembled, along with a brief review of the basic processes involved. These have been used to calculate dissociation and ionisation rates, with uncertainties, in a standard ultraviolet interstellar radiation field (ISRF) and wavelength-dependent radiation fields. The new ISRF rates generally agree within 30% with our previous compilations, with a few notable exceptions. The reduction of rates in shielded regions was calculated as a function of dust, molecular and atomic hydrogen, atomic C, and self-shielding column densities. The relative importance of shielding types depends on the species in question and the dust optical properties. The new data are publicly available from the Leiden photodissociation and ionisation database. Sensitivity of rates to variation of temperature and isotope, and cross section uncertainties, are tested. Tests were conducted with an interstellar-cloud chemical model, and find general agreement (within a factor of two) with the previous iteration of the Leiden database for the ISRF, and order-of-magnitude variations assuming various kinds of stellar radiation. The newly parameterised dust-shielding factors makes a factor-of-two difference to many atomic and molecular abundances relative to parameters currently in the UDfA and KIDA astrochemical reaction databases. The newly-calculated cosmic-ray induced photodissociation and ionisation rates differ from current standard values up to a factor of 5. Under high temperature and cosmic-ray-flux conditions the new rates alter the equilibrium abundances of abundant dark cloud abundances by up to a factor of two. The partial cross sections for H2O and NH3 photodissociation forming OH, O, NH2 and NH are also evaluated and lead to radiation-field-dependent branching ratios.Comment: Corrected some inconsistent table/figure data. Significant change: Zn photoionisation rate corrected. Accepted for publication by A&

Dense and warm molecular gas in the envelopes and outflows of southern low-mass protostars

Observations of dense molecular gas lie at the basis of our understanding of the density and temperature structure of protostellar envelopes and molecular outflows. We aim to characterize the properties of the protostellar envelope, molecular outflow and surrounding cloud, through observations of high excitation molecular lines within a sample of 16 southern sources presumed to be embedded YSOs. Observations of submillimeter lines of CO, HCO+ and their isotopologues, both single spectra and small maps were taken with the FLASH and APEX-2a instruments mounted on APEX to trace the gas around the sources. The HARP-B instrument on the JCMT was used to map IRAS 15398-3359 in these lines. HCO+ mapping probes the presence of dense centrally condensed gas, a characteristic of protostellar envelopes. The rare isotopologues C18O and H13CO+ are also included to determine the optical depth, column density, and source velocity. The combination of multiple CO transitions, such as 3-2, 4-3 and 7-6, allows to constrain outflow properties, in particular the temperature. Archival submillimeter continuum data are used to determine envelope masses. Eleven of the sixteen sources have associated warm and/or dense quiescent as characteristic of protostellar envelopes, or an associated outflow. Using the strength and degree of concentration of the HCO+ 4-3 and CO 4-3 lines as a diagnostic, five sources classified as Class I based on their spectral energy distributions are found not to be embedded YSOs. The C18O 3-2 lines show that for none of the sources, foreground cloud layers are present. Strong molecular outflows are found around six sources, .. (continued in paper)Comment: Accepted by A&A, 13 figure

Astrochemical confirmation of the rapid evolution of massive YSOs and explanation for the inferred ages of hot cores

Aims. To understand the roles of infall and protostellar evolution on the envelopes of massive young stellar objects (YSOs). Methods. The chemical evolution of gas and dust is traced, including infall and realistic source evolution. The temperatures are determined self-consistently. Both ad/desorption of ices using recent laboratory temperature-programmed-desorption measurements are included. Results. The observed water abundance jump near 100 K is reproduced by an evaporation front which moves outward as the luminosity increases. Ion-molecule reactions produce water below 100 K. The age of the source is constrained to t \~ 8 +/- 4 x 10^4 yrs since YSO formation. It is shown that the chemical age-dating of hot cores at ~ few x 10^3 - 10^4 yr and the disappearance of hot cores on a timescale of ~ 10^5 yr is a natural consequence of infall in a dynamic envelope and protostellar evolution. Dynamical structures of ~ 350AU such as disks should contain most of the complex second generation species. The assumed order of desorption kinetics does not affect these results.Comment: Accepted by A&A Letters; 4 pages, 5 figure

The deuterium fractionation of water on solar-system scales in deeply-embedded low-mass protostars

(Abridged) The water deuterium fractionation (HDO/H$_2$O abundance ratio) has traditionally been used to infer the amount of water brought to Earth by comets. Measuring this ratio in deeply-embedded low-mass protostars makes it possible to probe the critical stage when water is transported from clouds to disks in which icy bodies are formed. We present sub-arcsecond resolution observations of HDO in combination with H$_2^{18}$O from the PdBI toward the three low-mass protostars NGC 1333-IRAS 2A, IRAS 4A-NW, and IRAS 4B. The resulting HDO/H$_2$O ratio is $7.4\pm2.1\times10^{-4}$ for IRAS 2A, $19.1\pm5.4\times10^{-4}$ for IRAS 4A-NW, and $5.9\pm1.7\times10^{-4}$ for IRAS 4B. Derived ratios agree with radiative transfer models within a factor of 2-4 depending on the source. Our HDO/H$_2$O ratios for the inner regions (where $T>100$ K) of four young protostars are only a factor of 2 higher than those found for pristine, solar system comets. These small differences suggest that little processing of water occurs between the deeply embedded stage and the formation of planetesimals and comets.Comment: 10 pages, 6 figures, accepted for publication in Astronomy and Astrophysic

Chemistry and line emission from evolving Herbig Ae disks

Aims: To calculate chemistry and gas temperature of evolving protoplanetary disks with decreasing mass or dust settling, and to explore the sensitivity of gas-phase tracers. Methods: The density and dust temperature profiles for a range of models of flaring and self-shadowed disks around a typical Herbig Ae star are used together with 2-dimensional ultraviolet (UV) radiative transfer to calculate the chemistry and gas temperature. In each model the line profiles and intensities for the fine structure lines of [O I], [C II] and [C I] and the pure rotational lines of CO, CN, HCN and HCO+ are determined. Results: The chemistry shows a strong correlation with disk mass. Molecules that are easily dissociated, like HCN, require high densities and large extinctions before they can become abundant. The products of photodissociation, like CN and C2H, become abundant in models with lower masses. Dust settling mainly affects the gas temperature, and thus high temperature tracers like the O and C+ fine structure lines. The carbon chemistry is found to be very sensitive to the adopted PAH abundance. The line ratios CO/13CO, CO/HCO+ and [O I] 63 um/146 um can be used to distinguish between disks where dust growth and settling takes place, and disks that undergo overall mass loss.Comment: 14 pages, 12 figures. Accepted for publication in A&

Evidence for episodic warm outflowing CO gas from the intermediate mass young stellar object IRAS 08470-4321

We present a R=10,000 M-band spectrum of LLN19 (IRAS 08470-4321), a heavily embedded intermediate-mass young stellar object located in the Vela Molecular Cloud, obtained with VLT-ISAAC. The data were fitted by a 2-slab cold-hot model and a wind model. The spectrum exhibits deep broad ro-vibrational absorption lines of 12CO v=1<-0 and 13CO v=1<-0. A weak CO ice feature at 4.67 micron is also detected. Differences in velocity indicate that the warm gas is distinct from the cold millimeter emitting gas, which may be associated with the absorption by cooler gas (45K). The outflowing warm gas at 300-400K and with a mass-loss rate varying between 0.48E-7 and 4.2E-7 MSun /yr can explain most of the absorption. Several absorption lines were spectrally resolved in subsequent spectra obtained with the VLT-CRIRES instrument. Multiple absorption substructures in the high-resolution (R=100,000) spectra indicate that the mass-loss is episodic with at least two major events that occurred recently (<28 years). The discrete mass-loss events together with the large turbulent width of the gas (dv=10-12 km/s) are consistent with the predictions of the Jet-Bow shock outflow and the wide-angle wind model. The CO gas/solid column density ratio of 20-100 in the line-of-sight confirms that the circumstellar environment of LLN~19 is warm. We also derive a 12C/13C ratio of 67 +/- 3, consistent with previous measurements in local molecular clouds but not with the higher ratios found in the envelope of other young stellar objects.Comment: 16 pages, 12 figures, accepted for publication in MNRA

Formation rates of complex organics in UV irradiated CH3OH-rich ices I: Experiments

(Abridged) Gas-phase complex organic molecules are commonly detected in the warm inner regions of protostellar envelopes. Recent models show that photochemistry in ices followed by desorption may explain the observed abundances. This study aims to experimentally quantify the broad-band UV-induced production rates of complex organics in CH3OH-rich ices at 20-70 K under ultra-high vacuum conditions. The reaction products are mainly identified by RAIRS and TPD experiments. Complex organics are readily formed in all experiments, both during irradiation and during a slow warm-up of the ices to 200 K after the UV lamp is turned off. The relative abundances of photoproducts depend on the UV fluence, the ice temperature, and whether pure CH3OH ice or CH3OH:CH4/CO ice mixtures are used. C2H6, CH3CHO, CH3CH2OH, CH3OCH3, HCOOCH3, HOCH2CHO and (CH2OH)2 are all detected in at least one experiment. The derived product-formation yields and their dependences on different experimental parameters, such as the initial ice composition, are used to estimate the CH3OH photodissociation branching ratios in ice and the relative diffusion barriers of the formed radicals. The experiments show that ice photochemistry in CH3OH ices is efficient enough to explain the observed abundances of complex organics around protostars and that ratios of complex molecules can be used to constrain their formation pathway.Comment: Accepted for publication in A&A. 65 pages including appendice