arXiv subjects

F. F. S. van der Tak

Publications and source records attributed to F. F. S. van der Tak.

At least 109 records · Page 6Linked to original sources

arXiv2007

Tracing high energy radiation with molecular lines near deeply embedded protostars

Submillimeter lines of CN, NO, CO+ and SO+, and upper limits on SH+ and N2O are observed with the James Clerk Maxwell Telescope in two high-mass and up to nine low-mass young stellar objects and compared with chemical models. Constant fractional abundances derived from radiative transfer modeling of the line strengths are x(CN) ~ a few x 10^{-11}-10^{-8}, x(NO) ~ 10^{-9}-10^{-8} and x(CO+) ~ 10^{-12}-10^{-10}. SO+ has abundances of a few x 10^{-11} in the high-mass objects and upper limits of ~ 10^{-12}-10^{-11} in the low-mass sources. All abundances are up to 1-2 orders of magnitude higher if the molecular emission is assumed to originate mainly from the inner region (< 1000 AU) of the envelope. For high-mass sources, the CN, SO+ and CO+ abundances and abundance ratios are best explained by an enhanced far-ultraviolet (FUV) field impacting gas at temperatures of a few hundred K. The observed column densities require that this region of enhanced FUV has scales comparable to the observing beam, such as in a geometry in which the enhanced FUV irradiates outflow walls. For low-mass sources, the required temperatures within the FUV models of T > 300 K are much higher than found in models, so that an X-ray enhanced region close to the protostar (r < 500 AU) is more plausible. The observed abundances imply X-ray fluxes for the Class 0 objects of L_X ~ 10^{29}-10^{31} erg s^{-1}, comparable to those observed from low-mass Class I protostars. Spatially resolved data are needed to clearly distinguish the effects of FUV and X-rays for individual species.

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arXiv2006

The JCMT Spectral Legacy Survey

Stars form in the densest, coldest, most quiescent regions of molecular clouds. Molecules provide the only probes which can reveal the dynamics, physics, chemistry and evolution of these regions, but our understanding of the molecular inventory of sources and how this is related to their physical state and evolution is rudimentary and incomplete. The Spectral Legacy Survey (SLS) is one of seven surveys recently approved by the JCMT Board. Starting in 2007, the SLS will produce a spectral imaging survey of the content and distribution of all the molecules detected in the 345 GHz atmospheric window (between 332 GHz and 373 GHz) towards a sample of 5 sources. Our intended targets are: a low mass core (NGC1333 IRAS4), 3 high mass cores spanning a range of star forming environments and evolutionary states (W49, AFGL2591, and IRAS20126), and a PDR (the Orion Bar). The SLS will use the unique spectral imaging capabilities of HARP-B/ACSIS to study the molecular inventory and the physical structure of these objects, which span different evolutionary stages and physical environments, to probe their evolution during the star formation process. As its name suggests, the SLS will provide a lasting data legacy from the JCMT that is intended to benefit the entire astronomical community. As such, the entire data set (including calibrated spectral datacubes, maps of molecular emission, line identifications, and calculations of the gas temperature and column density) will be publicly available.

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arXiv2006

APEX mapping of H3O+ in the Sgr B2 region

The cosmic-ray ionization rate (zeta) of dense molecular clouds is a key parameter for their dynamics and chemistry. Variations of zeta are well established, but it is unclear if these are related to source column density or to Galactic location. Using the APEX telescope, we have mapped the 364 GHz line of H3O+ in the Sgr B2 region and observed the 307 GHz line at selected positions. With the IRAM 30-m telescope we have observed the 203 GHz line of H2O-18 at the same positions. Strong H3O+ emission is detected over a ~3x2 pc region, indicating H3O+ column densities of 10^15 - 10^16 cm^-2 in an 18" beam. The H3O+ abundance of ~3 x 10^-9 and H3O+/H2O ratio of ~1/50 in the Sgr B2 envelope are consistent with models with zeta ~4 x 10^-16 s^-1, 3x lower than derived from H3+ observations toward Sgr A, but 10x that of local dense clouds. The ionization rates of interstellar clouds thus seem to be to first order determined by the ambient cosmic-ray flux, while propagation effects cause a factor of ~3 decrease from diffuse to dense clouds.

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arXiv2006

Probable detection of H2D+ in the starless core Barnard 68

The presence of H2D+ in dense cloud cores underlies ion-molecule reactions that strongly enhance the deuterium fractionation of many molecular species. We determine the H2D+ abundance in one starless core, Barnard 68, that has a particularly well established physical, chemical, and dynamical structure. We observed the ortho-H2D+ ground-state line 1_10-1_11, the N2H+ J=4-3 line, and the H13CO+ 4-3 line with the APEX telescope. We report the probable detection of the o-H2D+ line at an intensity Tmb=0.22 +- 0.08 K and exclusively thermal line width, and find only upper limits to the N2H+ 4-3 and H13CO+ 4-3 intensities. Within the uncertainties in the chemical reaction rates and the collisional excitation rates, chemical model calculations and excitation simulations reproduce the observed intensities and that of o-H2D+ in particular.

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arXiv2006

The evolutionary state of the southern dense core Cha-MMS1

Aims: Our goal is to set constraints on the evolutionary state of the dense core Cha-MMS1 in the Chamaeleon I molecular cloud. Methods: We analyze molecular line observations carried out with the new submillimeter telescope APEX. We look for outflow signatures around the dense core and probe its chemical structure, which we compare to predictions of models of gas-phase chemistry. We also use the public database of the Spitzer Space Telescope (SST) to compare Cha-MMS1 with the two Class 0 protostars IRAM 04191 and L1521F, which are at the same distance. Results: We measure a large deuterium fractionation for N2H+ (11 +/- 3 %), intermediate between the prestellar core L1544 and the very young Class 0 protostar L1521F. It is larger than for HCO+ (2.5 +/- 0.9 %), which is probably the result of depletion removing HCO+ from the high-density inner region. Our CO(3-2) map reveals the presence of a bipolar outflow driven by the Class I protostar Ced 110 IRS 4 but we do not find evidence for an outflow powered by Cha-MMS1. We also report the detection of Cha-MMS1 at 24, 70 and 160 microns by the instrument MIPS of the SST, at a level nearly an order of magnitude lower than IRAM 04191 and L1521F. Conclusions: Cha-MMS1 appears to have already formed a compact object, either the first hydrostatic core at the very end of the prestellar phase, or an extremely young protostar that has not yet powered any outflow, at the very beginning of the Class 0 accretion phase.

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arXiv2006

The Thermal Structure of Gas in Pre-Stellar Cores: A Case Study of Barnard 68

We present a direct comparison of a chemical/physical model to multitransitional observations of C18O and 13CO towards the Barnard 68 pre-stellar core. These observations provide a sensitive test for models of low UV field photodissociation regions and offer the best constraint on the gas temperature of a pre-stellar core. We find that the gas temperature of this object is surprisingly low (~7-8 K), and significantly below the dust temperature, in the outer layers (Av < 5 mag) that are traced by C18O and 13CO emission. As shown previously, the inner layers (Av > 5 mag) exhibit significant freeze-out of CO onto grain surfaces. Because the dust and gas are not fully coupled, depletion of key coolants in the densest layers raises the core (gas) temperature, but only by ~1 K. The gas temperature in layers not traced by C18O and 13CO emission can be probed by NH3 emission, with a previously estimated temperature of ~10-11 K. To reach these temperatures in the inner core requires an order of magnitude reduction in the gas to dust coupling rate. This potentially argues for a lack of small grains in the densest gas, presumably due to grain coagulation.

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arXiv2006

Using H3+ and H2D+ as probes of star-forming regions

The H3+ and H2D+ ions are important probes of the physical and chemical conditions in regions of the interstellar medium where new stars are forming. This paper reviews how observations of these species and of heavier ions such as HCO+ and H3O+ can be used to derive chemical and kinematic properties of nearby pre-stellar cores, and the cosmic-ray ionisation rate toward more distant regions of high-mass star formation. Future prospects in the field are outlined at the end.

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arXiv2005

Recent Searches for the Radio Lines of NH_3 in Comets

Radio observations in the ammonia inversion lines of four comets, C/2001 A2 (LINEAR), 153P/Ikeya-Zhang, C/2001 Q4 (NEAT) and C/2002 T7 (LINEAR), were performed at the Effelsberg 100-m Radio Telescope during their respective close approaches to Earth. None of the four lowest energy metastable lines (J,K=J),J=1--4, could be detected in these comets. We derive the following 3 sigma upper bounds on the NH_3 production rate, and c omparing to the corresponding water production rates, percentage NH_3 abundances relative to H_2O: Q(NH_3) < 1.9x10^26 s^-1 (0.63%) for C/2001 A2 (LINEAR), Q(NH_3) < 2.7x10^26 s^-1 (0.13%) for C/2001 Q4 (NEAT), Q(NH_3) < 2.3x10^27 s^-1 (0.74%) for C/2002 T7 (LINEAR) and Q(NH_3) <= 6.3x10^26 s^-1 (0.63%) for Comet 153P/Ikeya-Zhang. At 0.74% or less, the ammonia-to-water ratios are factors of 2 below the value for C/1995 O1 (Hale-Bopp) and 1P/Halley, suggesting chemical diversity between comets. The 18-cm lines of OH were clearly detected in the two comets observed during the 2004 campaign, thereby validating the cometary ephemerides.

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arXiv2005

Line profiles of molecular ions toward the pre-stellar core LDN 1544

Velocity profiles of ground state lines of H2D+, HC18O+ and N2H+, observed previously with the CSO and IRAM 30m telescopes, are modeled with a Monte Carlo radiative transfer program to study the temperature, density and velocity structure of the pre-stellar core LDN 1544. The H2D+ line is double-peaked like that of the other ions, but previous models that fit the HC18O+ and N2H+ profiles are found not to fit the H2D+ data. Matching the H2D+ observations requires at least three modifications to the model at small radii: (1) the density profile must continue to rise inward and not flatten off toward the center; (2) the gas temperature must be nearly constant and not drop inwards significantly; (3) the infall velocity must increase inward, in a fashion intermediate between `quasi-static' (ambipolar diffusion) and `fully dynamic' (Larson-Penston) collapse. The C18O emission indicates a chemical age of <~0.1 Myr. The effects of a flattened structure and rotation on the line profiles are shown to be unimportant, at least on the scales probed by single-dish telescopes. Alternatively, the H2D+ profile is affected by absorption in the outer layers of the core, if gas motions in these layers are sufficiently small.

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arXiv2005

Interstellar deuterated ammonia: From NH3 to ND3

We use spectra and maps of NH2D, ND2H, and ND3, obtained with the CSO, IRAM 30m and Arecibo telescopes, to study deuteration processes in dense cores. The data include the first detection of the hyperfine structure of ND2H. The emission of ND2H and ND3 does not seem to peak at the positions of the embedded protostars, but instead at offset positions, where outflow interactions may occur. A constant ammonia fractionation ratio in star-forming regions is generally assumed to be consistent with an origin on dust grains. However, in the pre-stellar cores studied here, the fractionation varies significantly when going from NH3 to ND3. We present a steady state model of the gas-phase chemistry for these sources, which includes passive depletion onto dust grains and multiply saturated deuterated species up to five deuterium atoms (e.g. CD5+). The observed column density ratios of all four ammonia isotopologues are reproduced within a factor of 3 for a gas temperature of 10 K. We also predict that deuterium fractionation remains significant at temperatures up to 20 K. ND and NHD, which have rotational transitions in the submillimeter domain are predicted to be abundant.

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arXiv2004

Subarcsecond mid-infrared and radio observations of the W3 IRS5 protocluster

Observations at mid-infrared (4.8-17.65 um) and radio (0.7-1.3 cm) wavelengths are used to constrain the structure of the high-mass star-forming region W3 IRS5 on 0.1'' (200 AU) scales. Two bright mid-infrared sources are detected, as well as diffuse emission. The bright sources have associated compact radio emission and probably are young high-mass stars. The measured sizes and estimated temperatures indicate that these sources together can supply the observed far-infrared luminosity. However, an optically thick radio source with a possible mid-infrared counterpart may also contribute significant luminosity; if so, it must be extremely deeply embedded. The infrared colour temperatures of 350-390 K and low radio brightness suggest gravitational confinement of the H II regions and ongoing accretion at a rate of a few 10^-8 M0/yr or more. Variations in the accretion rate would explain the observed radio variability. The low estimated foreground extinction suggests the existence of a cavity around the central stars, perhaps blown by stellar winds. At least three radio sources without mid-infrared counterparts appear to show proper motions of ~100 km/s, and may be deeply embedded young runaway OB stars, but more likely are clumps in the ambient material which are shock-ionized by the OB star winds.

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arXiv2004

An atomic and molecular database for analysis of submillimetre line observations

Atomic and molecular data for the transitions of a number of astrophysically interesting species are summarized, including energy levels, statistical weights, Einstein A-coefficients and collisional rate coefficients. Available collisional data from quantum chemical calculations and experiments are extrapolated to higher energies. These data, which are made publically available through the WWW at http://www.strw.leidenuniv.nl/~moldata, are essential input for non-LTE line radiative transfer programs. An online version of a computer program for performing statistical equilibrium calculations is also made available as part of the database. Comparisons of calculated emission lines using different sets of collisional rate coefficients are presented. This database should form an important tool in analyzing observations from current and future (sub)millimetre and infrared telescopes.

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arXiv2003

Very compact radio emission from high-mass protostars. I. CRL 2136: Continuum and water maser observations

We report 5-43 GHz radio observations of the CRL 2136 region at 0.6-6 arcsec resolution. We detect weak (mJy intensity) radio emission from the deeply embedded high-mass protostar IRS 1, which has an optically thick spectrum up to frequencies of 22 GHz, flattening at higher frequencies, which might be explained by emission from a jet. Water maser mapping shows that the strong emission observed redshifted relative to the systemic velocity is spatially coincident with the optically thick continuum emission. The H2O maser emission from this object (and others we know of) seems to have a different origin than most of these masers, which are frequently tracing bipolar high-velocity outflows. Instead, the CRL 2136 H2O emission arises in the close circumstellar environment of the protostar (within 1000 AU). We speculate that most of it is excited in the hot, dense infalling gas after the accretion shock, although this cannot explain all the H2O emission. An accretion shock nature for the continuum emission seems unlikely.

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arXiv2003

Hot Molecular Cores and High-Mass Star Formation

This review covers hot cores in the context of high-mass star formation. After giving an overview of chemical processes and diversity during high-mass star formation, it reviews the `warm envelope' phase which probably precedes the formation of hot cores. Some recent determinations of the cosmic-ray ionization rate are discussed, as well as recent evidence for hot cores around low-mass stars. Routes for future hot core research are outlined.

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arXiv2003

Sulphur chemistry in the envelopes of massive young stars

The sulphur chemistry in nine regions in the earliest stages of high-mass star formation is studied through single-dish submillimeter spectroscopy. The line profiles indicate that 10-50% of the SO and SO2 emission arises in high-velocity gas, either infalling or outflowing. For the low-velocity gas, excitation temperatures are 25 K for H2S, 50 K for SO, H2CS, NS and HCS+, and 100 K for OCS and SO2, indicating that most observed emission traces the outer parts (T<100 K) of the molecular envelopes, except high-excitation OCS and SO2 lines. Abundances in the outer envelopes, calculated with a Monte Carlo program, using the physical structures of the sources derived from previous submillimeter continuum and CS line data, are ~10^-8 for OCS, ~10^-9 for H2S, H2CS, SO and SO2, and ~10^-10 for HCS+ and NS. In the inner envelopes (T>100 K) of six sources, the SO2 abundance is enhanced by factors of ~100-1000. This region of hot, abundant SO2 has been seen before in infrared absorption, and must be small, <~ 0.2 arcsec (180 AU radius). The derived abundance profiles are consistent with models of envelope chemistry which invoke ice evaporation at T~100$ K. Shock chemistry is unlikely to contribute. A major sulphur carrier in the ices is probably OCS, not H2S as most models assume. The source-to-source abundance variations of most molecules by factors of ~10 do not correlate with previous systematic tracers of envelope heating. Without observations of H2S and SO lines probing warm (>~ 100 K) gas, sulphur-bearing molecules cannot be used as evolutionary tracers during star formation.

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arXiv2003

Abundant H2D+ in the pre-stellar core L1544

We have detected the 372 GHz line of ortho-H2D+ towards the pre-stellar core L1544. The strongest emission (T_mb ~ 1 K) occurs at the peak of the millimeter continuum emission, while measurements at offset positions indicate that H2D+ is confined within ~ 20 arcsec, where CO is highly depleted. The derived H2D+ abundance of ~ 10^{-9} is comparable with previous estimates of the electron abundance in the core, which suggests that H2D+ is the main molecular ion in the central 20 arcsec (2800 AU) of L1544. This confirms the expectations that H2D+ is dramatically enhanced in gas depleted of molecules other than H2. The measured abundance even exceeds the present model predictions by about a factor ten. One possibility is that all CNO-bearing neutral species, including atomic oxygen, are almost completely (> 98%) frozen within a radius of ~2800 AU.

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arXiv2002

Sulphur chemistry in the envelopes of massive young stars

We present submillimeter observations of SO, SO2, H2S, H2CS, OCS, NS and HCS+ toward nine massive young stars. The outflow contributes ~50% to the SO and SO2 emission in 15-20'' beams, more than for CS, where it is 10%. The SO2 abundance increases from dark cloud levels in the outer envelope (T<100 K) to levels seen in hot cores and shocks in the inner envelope (T>100 K). Molecular abundances are consistent with a model of ice evaporation in an envelope with gradients in temperature and density for a chemical age of ~30000 yr. The high observed abundance of OCS, the fact that T_ex (OCS) >> T_ex (H2S), and the data on solid OCS and H2S all suggest that the major sulphur carrier in grain mantles is OCS rather than H2S. For most other sulphur-bearing molecules, the source-to-source abundance variations by factors of up to 10 do not correlate with previously established evolutionary trends in temperature tracers. These species probe the chemically inactive outer envelope. Our data set does not constrain the abundances of H2S and SO in the inner envelope, which, together with SO2, are required to use sulphur as a clock.

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arXiv2002

A database of molecular line data for rotational transitions from selected species of astrophysical interest

Molecular line data for the rotational transitions of a number of astrophysically interesting species have been collected and are made publically available through the www. These data, including energy levels, statistical weights, Einstein A-coefficients and collisional rate coefficients, are essential input for non-LTE molecular radiative transfer programs. A computer program for performing statistical equilibrium calculations is made publically available for use as well. This database should form an important tool in analyzing observations from current and future infrared and (sub)millimetre telescopes.

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