Detection of steam in the circumstellar disk around a massive Young Stellar Object

We report on the observation of hot water vapor (steam) in the inner AU of a young massive star located in the star-forming region IRAS 08576-4334. The water lines are detected in a medium resolution (R=10,000) K-band spectrum taken by the infrared spectrometer ISAAC mounted on the VLT-ANTU. The water vapor is at a mean temperature of 1565+/-510 K, cooler than the hot CO found in the same object, which is at 1660K and the column density is N(H2O)=(2.5 +/- 0.4)x1E18 cm-2. The profile of both H2O and CO lines is best reproduced by the emission from a Keplerian disk. To interpret the data, we also investigate the formation of molecules and especially CO and water vapor in the inner hot and dense part of disks around young high mass stars using a pseudo time-dependent gas-phase chemical model. Molecules are rapidly photodissociated but this destruction is compensated by an efficient formation due to fast neutral-neutral reactions. The ability of CO molecules to self-shield significantly enhances its abundance. Water molecules are sufficiently abundant to be detectable. The observed H2O/CO ratio is reproduced by gas at 1600K and an enhanced UV field over gas density ratio I_UV/nH=1E(-4)- 1E(-6). The simulations support the presence of CO and H2O molecules in the inner disks around young massive stars despite the strong UV radiation and show that the OH radical plays an essential role in hot gas chemistry.Comment: 15 pages, 11 figures, Accepted to A&

Chemical tracers in proto-brown dwarfs: CO, ortho-H2_{2}CO, para-H2_{2}CO, HCO+^{+}, CS observations

We present a study of the CO isotopologues and the high-density tracers H$_{2}$CO, HCO$^{+}$, and CS in Class 0/I proto-brown dwarfs (proto-BDs). We have used the IRAM 30m telescope to observe the $^{12}$CO (2-1), $^{13}$CO (2-1), C$^{18}$O (2-1), C$^{17}$O (2-1), H$_{2}$CO (3-2), HCO$^{+}$ (3-2), and CS (5-4) lines in 7 proto-BDs. The hydrogen column density for the proto-BDs derived from the CO gas emission is $\sim$2-15 times lower than that derived from the dust continuum emission, indicating CO depletion from the gas-phase. The mean H$_{2}$CO ortho-to-para ratio is $\sim$3 for the proto-BDs and indicates gas-phase formation for H$_{2}$CO. We have investigated the correlations in the molecular abundances between the proto-BDs and protostars. Proto-BDs on average show a factor of $\sim$2 higher ortho-to-para H$_{2}$CO ratio than the protostars. Possible explanations include a difference in the H$_{2}$CO formation mechanism, spin-selective photo-dissociation, self-shielding effects, or different emitting regions for the ortho and para species. There is a tentative trend of a decline in the HCO$^{+}$ and H$_{2}$CO abundances with decreasing bolometric luminosity, while the CS and CO abundances show no particular difference between the proto-BDs and protostars. These trends reflect the scaled-down physical structures for the proto-BDs compared to protostars and differences in the peak emitting regions for these species. The C$^{17}$O isotopologue is detected in all of the proto-BDs as well as the more evolved Class Flat/Class II BDs in our sample, and can probe the quiescent gas at both early and late evolutionary stages.Comment: Accepted in MNRAS. arXiv admin note: text overlap with arXiv:1809.1016

Chemical tracers in proto-brown dwarfs: CN, HCN, and HNC observations

We present results from a study of nitrogen chemistry in Class 0/I proto-brown dwarfs (proto-BDs). We have used the IRAM 30 m telescope to observe the CN (2-1), HCN (3-2), and HNC (3-2) lines in 7 proto-BDs. All proto-BDs show a large CN/HCN abundance ratio of >20, and a HNC/HCN abundance ratio close to or larger than unity. The enhanced CN/HCN ratios can be explained by high UV flux originating from an active accretion zone in the proto-BDs. The larger than unity HNC/HCN ratio for the proto-BDs is likely caused by a combination of low temperature and high density. Both CN and HNC show a flat distribution with CO, indicating that these species can survive in regions where CO is depleted. We have investigated the correlations in the molecular abundances of these species for the proto-BDs with Class 0/I protostars. We find tentative trends of CN (HCN) abundances being about an order of magnitude higher (lower) in the proto-BDs compared to protostars. HNC for the proto-BDs shows a nearly constant abundance unlike the large spread of ~2 orders of magnitude seen for the protostars. Also notable is a rise in the HNC/HCN abundance ratio for the lowest luminosity objects, suggesting that this ratio is higher under low-temperature environments. None of the relatively evolved Class Flat/Class II brown dwarfs in our sample show emission in HNC. The HNC molecule can be considered as an efficient tracer to search and identify early stage sub-stellar mass objects.Comment: Accepted in MNRA

Herschel-PACS observation of the 10 Myr old T Tauri disk TW Hya: Constraining the disk gas mass

Planets are formed in disks around young stars. With an age of ~10 Myr, TW Hya is one of the nearest T Tauri stars that is still surrounded by a relatively massive disk. In addition a large number of molecules has been found in the TWHya disk, making TWHya the perfect test case in a large survey of disks with Herschel–PACS to directly study their gaseous component. We aim to constrain the gas and dust mass of the circumstellar disk around TW Hya. We observed the fine-structure lines of [O_I] and [C_(II)] as part of the open-time large program GASPS. We complement this with continuum data and ground-based ^(12)CO 3–2 and ^(13)CO 3–2 observations. We simultaneously model the continuum and the line fluxes with the 3D Monte-Carlo code MCFOST and the thermo-chemical code ProDiMo to derive the gas and dust masses. We detect the [O_I] line at 63 μm. The other lines that were observed, [O_I] at 145 μm and [C_(II)] at 157 μm, are not detected. No extended emission has been found. Preliminary modeling of the photometric and line data assuming [^(12)CO]/[^(13)CO] = 69 suggests a dust mass for grains with radius <1 mm of ~1.9 × 10^(−4) M_⊙ (total solid mass of 3 × 10^(−3) M_⊙) and a gas mass of (0.5–5) ×10^(−3) M_⊙. The gas-to-dust mass may be lower than the standard interstellar value of 100

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

Evidence for an inner molecular disk around massive Young Stellar Objects

We present observations of CO overtone bandhead emission toward four massive Young Stellar Objects (spectral type O6--B5). The high signal-to-noise ratio K-band spectra were obtained with VLT-ISAAC at a resolution of 30 km/s, sufficient to resolve the bandheads, but not the individual J-lines. We are able to explain the shape of the lines by assuming a simple isothermal keplerian disk model seen at different inclinations. The gas temperature ranges from 1500 to 4500 K and the CO column density is between 0.1 and 4 10^21 cm^-2. The emission probably arises within the first few astronomical units of the disk, consistent with the high gas temperature. Our results indicate that molecules can survive close to a hot star and suggest that dense (n_H > 10^10 cm^-3) inner disks may be relatively common at an advanced stage of high-mass star formation.Comment: accepted for publication in Astronomy and Astrophysics Letter

The effects of dust evolution on disks in the mid-IR

In this paper, we couple together the dust evolution code two-pop-py with the thermochemical disk modelling code ProDiMo. We create a series of thermochemical disk models that simulate the evolution of dust over time from 0.018 Myr to 10 Myr, including the radial drift, growth, and settling of dust grains. We examine the effects of this dust evolution on the mid-infrared gas emission, focussing on the mid-infrared spectral lines of C2H2, CO2, HCN, NH3, OH, and H2O that are readily observable with Spitzer and the upcoming E-ELT and JWST. The addition of dust evolution acts to increase line fluxes by reducing the population of small dust grains. We find that the spectral lines of all species except C2H2 respond strongly to dust evolution, with line fluxes increasing by more than an order of magnitude across the model series as the density of small dust grains decreases over time. The C2H2 line fluxes are extremely low due to a lack of abundance in the infrared line-emitting regions, despite C2H2 being commonly detected with Spitzer, suggesting that warm chemistry in the inner disk may need further investigation. Finally, we find that the CO2 flux densities increase more rapidly than the other species as the dust disk evolves. This suggests that the flux ratios of CO2 to other species may be lower in disks with less-evolved dust populations.Comment: 13 pages, 9 figures, accepted in A&

Results from a VLT-ISAAC survey of ices and gas around young stellar objects

General results from a 3-5 micron spectroscopic survey of nearby low-mass young stellar objects are presented. L and M-band spectra have been obtained of \~50 low mass embedded young stars using the ISAAC spectrometer mounted on UT1-Antu at Paranal Observatory. For the first time, a consistent census of the CO, H2O ices and the minor ice species CH3OH and OCN- and warm CO gas present around young stars is obtained, using large number statistics and resolving powers of up to R=10000. The molecular structure of circumstellar CO ices, the depletion of gaseous CO onto grains in protoplanetary disks, the presence of hot gas in the inner parts of circumstellar disks and in outflows and infalls are studied. Furthermore, the importance of scattering effects for the interpretation of the spectra have been addressed.Comment: To appear in the proceedings of the conference "Chemistry as a Diagnostic of Star Formation", University of Waterloo, Canada, 21-23 August 200

FUV and X-ray irradiated protoplanetary disks: a grid of models I. The disk structure

Context. Planets are thought to eventually form from the mostly gaseous (~99% of the mass) disks around young stars. The density structure and chemical composition of protoplanetary disks are affected by the incident radiation field at optical, FUV, and X-ray wavelengths, as well as by the dust properties. Aims. The effect of FUV and X-rays on the disk structure and the gas chemical composition are investigated. This work forms the basis of a second paper, which discusses the impact on diagnostic lines of, e.g., C+, O, H2O, and Ne+ observed with facilities such as Spitzer and Herschel. Methods. A grid of 240 models is computed in which the X-ray and FUV luminosity, minimum grain size, dust size distribution, and surface density distribution are varied in a systematic way. The hydrostatic structure and the thermo-chemical structure are calculated using ProDiMo. Results. The abundance structure of neutral oxygen is stable to changes in the X-ray and FUV luminosity, and the emission lines will thus be useful tracers of the disk mass and temperature. The C+ abundance distribution is sensitive to both X-rays and FUV. The radial column density profile shows two peaks, one at the inner rim and a second one at a radius r=5-10 AU. Ne+ and other heavy elements have a very strong response to X-rays, and the column density in the inner disk increases by two orders of magnitude from the lowest (LX = 1e29 erg/s) to the highest considered X-ray flux (LX = 1e32 erg/s). FUV confines the Ne+ ionized region to areas closer to the star at low X-ray luminosities (LX = 1e29 erg/s). H2O abundances are enhanced by X-rays due to higher temperatures in the inner disk and higher ionization fractions in the outer disk. The line fluxes and profiles are affected by the effects on these species, thus providing diagnostic value in the study of FUV and X-ray irradiated disks around T Tauri stars. (abridged)Comment: 47 pages, accepted by Astronomy and Astrophysics, a high resolution version of the paper is located at http://www.astro.rug.nl/~meijerink/disk_paperI_xrays.pd