F. F. S. van der Tak

University of Groningen, Groningen, Groningen, Netherlands

Are you F. F. S. van der Tak?

Claim your profile

Publications (121)340.51 Total impact

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Water probes the dynamics in young stellar objects (YSOs) effectively, especially shocks in molecular outflows. It is a key molecule for exploring whether the physical properties of low-mass protostars can be extrapolated to massive YSOs. As part of the WISH key programme, we investigate the dynamics and the excitation conditions of shocks along the outflow cavity wall as function of source luminosity. Velocity-resolved Herschel-HIFI spectra of the H2O 988, 752, 1097 GHz and 12CO J=10-9, 16-15 lines were analysed for 52 YSOs with bolometric luminosities (L_bol) ranging from <1 to >10^5 L_sun. The profiles of the H2O lines are similar, indicating that they probe the same gas. We see two main Gaussian emission components in all YSOs: a broad component associated with non-dissociative shocks in the outflow cavity wall (cavity shocks) and a narrow component associated with quiescent envelope material. More than 60% of the total integrated intensity of the H2O lines (L_H2O) comes from the cavity shock component. The H2O line widths are similar for all YSOs, whereas those of 12CO 10-9 increase slightly with L_bol. The excitation analysis of the cavity shock component, performed with the non-LTE radiative transfer code RADEX, shows stronger 752 GHz emission for high-mass YSOs, likely due to pumping by an infrared radiation field. As previously found for CO, a strong correlation with slope unity is measured between log(L_H2O) and log(L_bol), which can be extrapolated to extragalactic sources. We conclude that the broad component of H2O and high-J CO lines originate in shocks in the outflow cavity walls for all YSOs, whereas lower-J CO transitions mostly trace entrained outflow gas. The higher UV field and turbulent motions in high-mass objects compared to their low-mass counterparts may explain the slightly different kinematical properties of 12CO 10-9 and H2O lines from low- to high-mass YSOs.
    Preview · Article · Nov 2015 · Astronomy and Astrophysics
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Context: Infrared dark clouds are the coldest and densest portions of giant molecular clouds. The most massive ones represent some of the most likely birthplaces for the next generation of massive stars in the Milky Way. Because a strong mid-IR background is needed to make them appear in absorption, they are usually assumed to be nearby. Aims: We use THz absorption spectroscopy to solve the distance ambiguity associated with kinematic distances for the IR-dark clouds in the TOP100 ATLASGAL sample, a flux-limited selection of massive clumps in different evolutionary phases of star formation. Methods: The para-H2O ground state transition at 1113.343 GHz, observed with Herschel/HIFI, was used to investigate the occurrence of foreground absorption along the line of sight directly towards infrared-dark clouds. Additional consistency checks were performed using MALT90 and HiGAL archival data and targeted Mopra and APEX spectroscopic observations. Results: We report the first discovery of five IRDCs in the TOP100 lying conclusively at the far kinematic distance, showing that the mere presence of low-contrast mid-IR absorption is not sufficient to unequivocally resolve the near/far ambiguity in favour of the former. All IRDCs are massive and actively forming high-mass stars; four of them also show infall signatures. Conclusions: We give a first estimate of the fraction of dark sources at the far distance (~11% in the TOP100) and describe their appearance and properties. The assumption that all dark clouds lie at the near distance may lead, in some cases, to underestimating masses, sizes, and luminosities, possibly causing clouds to be missed that will form very massive stars and clusters.
    Full-text · Article · Jul 2015 · Astronomy and Astrophysics
  • Source
    E. Koumpia · P. M. Harvey · V. Ossenkopf · F. F. S. van der Tak · B. Mookerjea · A. Fuente · C. Kramer
    [Show abstract] [Hide abstract]
    ABSTRACT: In dense parts of interstellar clouds (> 10^5 cm^-3), dust & gas are expected to be in thermal equilibrium, being coupled via collisions. However, previous studies have shown that the temperatures of the dust & gas may remain decoupled even at higher densities. We study in detail the temperatures of dust & gas in the photon-dominated region S 140, especially around the deeply embedded infrared sources IRS 1-3 and at the ionization front. We derive the dust temperature and column density by combining Herschel PACS continuum observations with SOFIA observations at 37 $\mu$m and SCUBA at 450 $\mu$m. We model these observations using greybody fits and the DUSTY radiative transfer code. For the gas part we use RADEX to model the CO 1-0, CO 2-1, 13CO 1-0 and C18O 1-0 emission lines mapped with the IRAM-30m over a 4' field. Around IRS 1-3, we use HIFI observations of single-points and cuts in CO 9-8, 13CO 10-9 and C18O 9-8 to constrain the amount of warm gas, using the best fitting dust model derived with DUSTY as input to the non-local radiative transfer model RATRAN. We find that the gas temperature around the infrared sources varies between 35 and 55K and that the gas is systematically warmer than the dust by ~5-15K despite the high gas density. In addition we observe an increase of the gas temperature from 30-35K in the surrounding up to 40-45K towards the ionization front, most likely due to the UV radiation from the external star. Furthermore, detailed models of the temperature structure close to IRS 1 show that the gas is warmer and/or denser than what we model. Finally, modelling of the dust emission from the sub-mm peak SMM 1 constrains its luminosity to a few ~10^2 Lo. We conclude that the gas heating in the S 140 region is very efficient even at high densities, most likely due to the deep UV penetration from the embedded sources in a clumpy medium and/or oblique shocks.
    Full-text · Article · Apr 2015 · Astronomy and Astrophysics
  • [Show abstract] [Hide abstract]
    ABSTRACT: In diffuse interstellar clouds the chemistry that leads to the formation of the oxygen-bearing ions OH^+, H_2O^+, and H_3O^+ begins with the ionization of atomic hydrogen by cosmic rays, and continues through subsequent hydrogen abstraction reactions involving H_2. Given these reaction pathways, the observed abundances of these molecules are useful in constraining both the total cosmic-ray ionization rate of atomic hydrogen (ζ_H) and molecular hydrogen fraction (ƒ_H_2). We present observations targeting transitions of OH^+, H_2O^+, and H_3O^+ made with the Herschel Space Observatory along 20 Galactic sight lines toward bright submillimeter continuum sources. Both OH^+ and H_2O^+ are detected in absorption in multiple velocity components along every sight line, but H_3O^+ is only detected along 7 sight lines. From the molecular abundances we compute ƒ_H_2 in multiple distinct components along each line of sight, and find a Gaussian distribution with mean and standard deviation 0.042 ± 0.018. This confirms previous findings that OH^+ and H_2O^+ primarily reside in gas with low H_2 fractions. We also infer ζ_H throughout our sample, and find a lognormal distribution with mean log (ζ_H) = –15.75 (ζ_H = 1.78 × 10^(–16) s^(–1)) and standard deviation 0.29 for gas within the Galactic disk, but outside of the Galactic center. This is in good agreement with the mean and distribution of cosmic-ray ionization rates previously inferred from H_3^+ observations. Ionization rates in the Galactic center tend to be 10-100 times larger than found in the Galactic disk, also in accord with prior studies.
    No preview · Article · Feb 2015
  • Source
    Z. Nagy · F. F. S. van der Tak · G. A. Fuller · R. Plume
    [Show abstract] [Hide abstract]
    ABSTRACT: The massive and luminous star-forming region W49A is a well known Galactic candidate to probe the physical conditions and chemistry similar to those expected in external starburst galaxies. We aim to probe the physical and chemical structure of W49A on a spatial scale of ~0.8 pc based on the JCMT Spectral Legacy Survey, which covers the frequency range between 330 and 373 GHz. The wide 2x2 arcminutes field and the high spectral resolution of the HARP instrument on JCMT provides information on the spatial structure and kinematics of the cloud. For species where multiple transitions are available, we estimate excitation temperatures and column densities. We detected 255 transitions corresponding to 60 species in the 330-373 GHz range at the center position of W49A. Excitation conditions can be probed for 16 molecules. The chemical composition suggests the importance of shock-, PDR-, and hot core chemistry. Many molecular lines show a significant spatial extent across the maps including high density tracers (e.g. HCN, HNC, CS, HCO+) and tracers of UV-irradiation (e.g. CN and C2H). Large variations are seen between the sub-regions with mostly blue-shifted emission toward the Eastern tail, mostly red-shifted emission toward the Northern clump, and emission peaking around the expected source velocity toward the South-west clump. A comparison of column density ratios of characteristic species observed toward W49A to Galactic PDRs suggests that while the chemistry toward the W49A center is driven by a combination of UV-irradiation and shocks, UV-irradiation dominates for the Northern Clump, Eastern tail, and South-west clump regions. A comparison to a starburst galaxy and an AGN suggests similar C2H, CN, and H2CO abundances (with respect to the dense gas tracer 34CS) between the ~0.8 pc scale probed for W49A and the >1 kpc regions in external galaxies with global star-formation.
    Full-text · Article · Feb 2015 · Astronomy and Astrophysics
  • Source
    Y. Choi · F. F. S. van der Tak · E. F. van Dishoeck · Fabrice Herpin · F. Wyrowski
    [Show abstract] [Hide abstract]
    ABSTRACT: Water is a sensitive tracer of physical conditions in star-forming regions because of its large abundance variations between hot and cold regions. We use spectrally resolved observations of rotational lines of H$_2$O and its isotopologs with Herschel/HIFI to constrain the physical conditions of the water emitting region toward the high-mass protostar AFGL2591. We use analytical estimates and rotation diagrams to estimate T$_{ex}$ and column densities of H$_2$O of the envelope, the outflow, and a foreground cloud. Furthermore, we use the non-LTE radiative transfer code to estimate the temperature and volume density of the H$_2$O emitting gas. Assuming LTE, we estimate an T$_{ex}$ of 42 K and a column density of 2$\times$10$^{14}$ cm$^{-2}$ for the envelope and 45 K and 4$\times$10$^{13}$ cm$^{-2}$ for the outflow, in beams of 4" and 30", respectively. Non-LTE models indicate a kinetic temperature of 60-230 K and a volume density of 7$\times$10$^6$-10$^8$ cm$^{-3}$ for the envelope, and a kinetic temperature of 70-90 K and a gas density of 10$^7$-10$^8$ cm$^{-3}$ for the outflow. The o/p ratio of the foreground absorption is 1.9$\pm$0.4, suggesting a low temperature. In contrast, the o/p ratio seen in absorption by the outflow is 3.5$\pm$1.0, as expected for warm gas. The water abundance in the envelope is 10$^{-9}$, similar to the low values found for other high- and low-mass protostars, suggesting that this abundance is constant during the embedded phase of high-mass star formation. The water abundance in the outflow is 10$^{-10}$, which is 10$\times$ lower than in the envelope and in the outflows of high- and low-mass protostars. Since beam size effects can only increase this estimate by a factor of 2, we suggest that the water in the outflow is affected by dissociating UV radiation due to the low extinction in the outflow lobe.
    Preview · Article · Dec 2014 · Astronomy and Astrophysics
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: In diffuse interstellar clouds the chemistry that leads to the formation of the oxygen bearing ions OH+, H2O+, and H3O+ begins with the ionization of atomic hydrogen by cosmic rays, and continues through subsequent hydrogen abstraction reactions involving H2. Given these reaction pathways, the observed abundances of these molecules are useful in constraining both the total cosmic-ray ionization rate of atomic hydrogen (zeta_H) and molecular hydrogen fraction, f(H2). We present observations targeting transitions of OH+, H2O+, and H3O+ made with the Herschel Space Observatory along 20 Galactic sight lines toward bright submillimeter continuum sources. Both OH+ and H2O+ are detected in absorption in multiple velocity components along every sight line, but H3O+ is only detected along 7 sight lines. From the molecular abundances we compute f(H2) in multiple distinct components along each line of sight, and find a Gaussian distribution with mean and standard deviation 0.042+-0.018. This confirms previous findings that OH+ and H2O+ primarily reside in gas with low H2 fractions. We also infer zeta_H throughout our sample, and find a log-normal distribution with mean log(zeta_H)=-15.75, (zeta_H=1.78x10^-16 s^-1), and standard deviation 0.29 for gas within the Galactic disk, but outside of the Galactic center. This is in good agreement with the mean and distribution of cosmic-ray ionization rates previously inferred from H3+ observations. Ionization rates in the Galactic center tend to be 10--100 times larger than found in the Galactic disk, also in accord with prior studies.
    Full-text · Article · Dec 2014 · The Astrophysical Journal
  • [Show abstract] [Hide abstract]
    ABSTRACT: VizieR On-line Data Catalog: J/A+A/572/A21. Originally published in: 2014A&A...572A..21M
    No preview · Article · Nov 2014
  • [Show abstract] [Hide abstract]
    ABSTRACT: Context. Theoretical scenarios propose that high-mass stars are formed by disk-mediated accretion. Aims. To test the theoretical predictions on the formation of massive stars, we wish to make a thorough study at high-angular resolution of the structure and kinematics of the dust and gas emission toward the high-mass star-forming region G35.03+0.35, which harbors a disk candidate around a B-type (proto)star. Methods. We carried out ALMA Cycle 0 observations at 870 μm of dust of typical high-density, molecular outflow, and cloud tracers with resolutions of 10^7 cm^(-3), and masses in the range 1–5 M_⊙, and they are subcritical. Core A, which is associated with a hypercompact Hii region and could be the driving source of the molecular outflow observed in the region, is the most chemically rich source in G35.03+0.35 with strong emission of typical hot core tracers such as CH_3CN. Tracers of high density and excitation show a clear velocity gradient along the major axis of the core, which is consistent with a disk rotating about the axis of the associated outflow. The PV plots along the SE–NW direction of the velocity gradient show clear signatures of Keplerian rotation, although infall could also be present, and they are consistent with the pattern of an edge-on Keplerian disk rotating about a star with a mass in the range 5–13 M_⊙. The high t_(ff)/t_(rot) ratio for core A suggests that the structure rotates fast and that the accreting material has time to settle into a centrifugally supported disk. Conclusions. G35.03+0.35 is one of the most convincing examples of Keplerian disks rotating about high-mass (proto)stars. This supports theoretical scenarios according to which high-mass stars, at least B-type stars, would form through disk-mediated accretion.
    No preview · Article · Nov 2014 · Astronomy and Astrophysics
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Context: Outflows are an important part of the star formation process as both the result of ongoing active accretion and one of the main sources of mechanical feedback on small scales. Water is the ideal tracer of these effects because it is present in high abundance in various parts of the protostar. Method: We present \textit{Herschel} HIFI spectra of multiple water-transitions towards 29 nearby Class 0/I protostars as part of the WISH Survey. These are decomposed into different Gaussian components, with each related to one of three parts of the protostellar system; quiescent envelope, cavity shock and spot shocks in the jet and at the base of the outflow. We then constrain the excitation conditions present in the two outflow-related components. Results: Water emission is optically thick but effectively thin, with line ratios that do not vary with velocity, in contrast to CO. The physical conditions of the cavity and spot shocks are similar, with post-shock H$_{2}$ densities of order 10$^{5}-$10$^{8}$\,cm$^{-3}$ and H$_{2}$O column densities of order 10$^{16}-$10$^{18}$\,cm$^{-2}$. H$_{2}$O emission originates in compact emitting regions: for the spot shocks these correspond to point sources with radii of order 10-200\,AU, while for the cavity shocks these come from a thin layer along the outflow cavity wall with thickness of order 1-30\,AU. Conclusions: Water emission at the source position traces two distinct kinematic components in the outflow; J shocks at the base of the outflow or in the jet, and C shocks in a thin layer in the cavity wall. Class I sources have similar excitation conditions to Class 0 sources, but generally smaller line-widths and emitting region sizes. We suggest that it is the velocity of the wind driving the outflow, rather than the decrease in envelope density or mass, that is the cause of the decrease in H$_{2}$O intensity between Class 0 and I.
    Full-text · Article · Sep 2014 · Astronomy and Astrophysics
  • Source
    Y. Choi · F. F. S. van der Tak · E. A. Bergin · R. Plume
    [Show abstract] [Hide abstract]
    ABSTRACT: The ortho-to-para ratio (OPR) of H$_2$O is thought to be sensitive to the temperature of water formation. The OPR of H$_2$O is thus useful to study the formation mechanism of water. We investigate the OPR of water in the Orion PDR (Photon-dominated region), at the Orion Bar and Orion S positions, using data from {\it Herschel}/HIFI. We detect the ground-state lines of ortho- and para-H$_2$$^{18}$O in the Orion Bar and Orion S and we estimate the column densities using LTE and non-LTE methods. Based on our calculations, the ortho-to-para ratio (OPR) in the Orion Bar is 0.1 $-$ 0.5, which is unexpectedly low given the gas temperature of $\sim$ 85 K, and also lower than the values measured for other interstellar clouds and protoplanetary disks. Toward Orion S, our OPR estimate is below 2. This low OPR at 2 positions in the Orion PDR is inconsistent with gas phase formation and with thermal evaporation from dust grains, but it may be explained by photodesorption.
    Full-text · Article · Sep 2014 · Astronomy and Astrophysics
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Context. The formation process of high-mass stars (with masses >8 M_⊙) is still poorly understood, and represents a challenge from both the theoretical and observational points of view. The advent of the Atacama Large Millimeter Array (ALMA) is expected to provide observational evidence to better constrain the theoretical scenarios. Aims. The present study aims at characterizing the high-mass star forming region G35.20−0.74 N, which is found associated with at least one massive outflow and contains multiple dense cores, one of them recently found associated with a Keplerian rotating disk. Methods. We used the radio-interferometer ALMA to observe the G35.20−0.74 N region in the submillimeter continuum and line emission at 350 GHz. The observed frequency range covers tracers of dense gas (e.g., H^(13)CO^+, C^(17)O), molecular outflows (e.g., SiO), and hot cores (e.g., CH_3CN, CH_3OH). These observations were complemented with infrared and centimeter data. Results. The ALMA 870 μm continuum emission map reveals an elongated dust structure (~0.15 pc long and ~0.013 pc wide; full width at half maximum) perpendicular to the large-scale molecular outflow detected in the region, and fragmented into a number of cores with masses ~1–10 M_⊙ and sizes ~1600 AU (spatial resolution ~960 AU). The cores appear regularly spaced with a separation of ~0.023 pc. The emission of dense gas tracers such as H^(13)CO^+ or C^(17)O is extended and coincident with the dust elongated structure. The three strongest dust cores show emission of complex organic molecules characteristic of hot cores, with temperatures around 200 K, and relative abundances 0.2–2 × 10^(-8) for CH_3CN and 0.6–5 × 10^(-6) for CH_3OH. The two cores with highest mass (cores A and B) show coherent velocity fields, with gradients almost aligned with the dust elongated structure. Those velocity gradients are consistent with Keplerian disks rotating about central masses of 4–18 M_⊙. Perpendicular to the velocity gradients we have identified a large-scale precessing jet/outflow associated with core B, and hints of an east-west jet/outflow associated with core A. Conclusions. The elongated dust structure in G35.20−0.74 N is fragmented into a number of dense cores that may form high-mass stars. Based on the velocity field of the dense gas, the orientation of the magnetic field, and the regularly spaced fragmentation, we interpret this elongated structure as the densest part of a 1D filament fragmenting and forming high-mass stars.
    Full-text · Article · Jun 2014 · Astronomy and Astrophysics
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: This paper presents the richness of submillimeter spectral features in the high-mass star forming region AFGL 2591. As part of the CHESS (Chemical Herschel Survey of Star Forming Regions) Key Programme, AFGL 2591 was observed by the Herschel/HIFI instrument. The spectral survey covered a frequency range from 480 up to 1240 GHz as well as single lines from 1267 to 1901 GHz (i.e. CO, HCl, NH3, OH and [CII]). Rotational and population diagram methods were used to calculate column densities, excitation temperatures and the emission extents of the observed molecules associated with AFGL 2591. The analysis was supplemented with several lines from ground-based JCMT spectra. From the HIFI spectral survey analysis a total of 32 species were identified (including isotopologues). In spite of the fact that lines are mostly quite week, 268 emission and 16 absorption lines were found (excluding blends). Molecular column densities range from 6e11 to 1e19 cm-2 and excitation temperatures range from 19 to 175 K. One can distinguish cold (e.g. HCN, H2S, NH3 with temperatures below 70 K) and warm species (e.g. CH3OH, SO2) in the protostellar envelope.
    Preview · Article · May 2014 · Astronomy and Astrophysics
  • Source
    Z. Nagy · V. Ossenkopf · F. F. S. Van der Tak · A. Faure · Z. Makai · E. A. Bergin
    [Show abstract] [Hide abstract]
    ABSTRACT: C$_2$H is one of the first radicals to be detected in the interstellar medium. Its higher rotational transitions have recently become available with the Herschel Space Observatory. We aim to constrain the physical parameters of the C$_2$H emitting gas toward the Orion Bar. We analyse the C$_2$H line intensities measured toward the Orion Bar CO$^+$ Peak and Herschel/HIFI maps of C$_2$H, CH, and HCO$^+$, and a NANTEN map of [CI]. We interpret the observed C$_2$H emission using radiative transfer and PDR models. Five rotational transitions of C$_2$H have been detected in the HIFI frequency range toward the CO$^+$ peak. A single component rotational diagram gives a rotation temperature of ~64 K and a beam-averaged C$_2$H column density of 4$\times$10$^{13}$ cm$^{-2}$. The measured transitions cannot be explained by any single parameter model. According to a non-LTE model, most of the C$_2$H column density produces the lower-$N$ C$_2$H transitions and traces a warm ($T_{\rm{kin}}$ ~ 100-150 K) and dense ($n$(H$_2$)~10$^5$-10$^6$ cm$^{-3}$) gas. A small fraction of the C$_2$H column density is required to reproduce the intensity of the highest-$N$ transitions ($N$=9-8 and N=10-9) originating from a high density ($n$(H$_2$)~5$\times$10$^6$ cm$^{-3}$) hot ($T_{\rm{kin}}$ ~ 400 K) gas. The total beam-averaged C$_2$H column density in the model is 10$^{14}$ cm$^{-2}$. Both the non-LTE radiative transfer model and a simple PDR model representing the Orion Bar with a plane-parallel slab of gas and dust suggest, that C$_2$H cannot be described by a single pressure component, unlike the reactive ion CH$^+$, which was previously analysed toward the Orion Bar CO$^+$ peak. The physical parameters traced by the higher rotational transitions ($N$=6-5,...,10-9) of C$_2$H may be consistent with the edges of dense clumps exposed to UV radiation near the ionization front of the Orion Bar.
    Full-text · Article · May 2014 · Astronomy and Astrophysics
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: (Abridged) We study the response of the gas to energetic processes associated with high-mass star formation and compare it with studies on low- and intermediate-mass young stellar objects (YSOs) using the same methods. The far-IR line emission and absorption of CO, H$_2$O, OH, and [OI] reveals the excitation and the relative contribution of different species to the gas cooling budget. Herschel-PACS spectra covering 55-190 um are analyzed for ten high-mass star forming regions of various luminosities and evolutionary stages at spatial scales of ~10^4 AU. Radiative transfer models are used to determine the contribution of the envelope to the far-IR CO emission. The close environments of high-mass YSOs show strong far-IR emission from molecules, atoms, and ions. Water is detected in all 10 objects even up to high excitation lines. CO lines from J=14-13 up to typically 29-28 show a single temperature component, Trot~300 K. Typical H$_2$O temperatures are Trot~250 K, while OH has Trot~80 K. Far-IR line cooling is dominated by CO (~75 %) and to a smaller extent by OI (~20 %), which increases for the most evolved sources. H$_2$O is less important as a coolant for high-mass sources because many lines are in absorption. Emission from the envelope is responsible for ~45-85 % of the total CO luminosity in high-mass sources compared with only ~10 % for low-mass YSOs. The highest-J lines originate most likely from shocks, based on the strong correlation of CO and H$_2$O with physical parameters of the sources from low- to high-masses. Excitation of warm CO is very similar for all mass regimes, whereas H$_2$O temperatures are ~100 K higher for high-mass sources than the low-mass YSOs. Molecular cooling is ~4 times more important than cooling by [OI]. The total far-IR line luminosity is about 10$^{-3}$ and 10$^{-5}$ times lower than the dust luminosity for the low- and high-mass YSOs.
    Full-text · Article · Nov 2013 · Astronomy and Astrophysics
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Aims. The aim of this paper is to study deuterated water in the solar-type protostars NGC1333 IRAS4A and IRAS4B, compare their HDO abundance distribution with other star-forming regions and constrain their HDO/H2O ratios. Methods. Using the Herschel/HIFI instrument as well as ground-based telescopes, we observed several HDO lines covering a large excitation range (Eup/k=22-168 K) towards these protostars and an outflow position. Non-LTE radiative transfer codes were then used to determine the HDO abundance profiles in these sources. Results. The HDO fundamental line profiles show a very broad component, tracing the molecular outflows, in addition to a narrower emission component as well as a narrow absorbing component. In the protostellar envelope of NGC1333 IRAS4A, the HDO inner (T>100 K) and outer (T<100 K) abundances with respect to H2 are estimated at 7.5x10^{-9} and 1.2x10^{-11} respectively, whereas, in NGC1333 IRAS4B, they are 1.0x10^{-8} and 1.2x10^{-10} respectively. Similarly to the low-mass protostar IRAS16293-2422, an absorbing outer layer with an enhanced abundance of deuterated water is required to reproduce the absorbing components seen in the fundamental lines at 465 and 894 GHz in both sources. This water-rich layer is probably extended enough to encompass the two sources as well as parts of the outflows. In the outflows emanating from NGC1333 IRAS4A, the HDO column density is estimated at about (2-4)x10^{13} cm^{-2}, leading to an abundance of about (0.7-1.9)x10^{-9}. An HDO/H2O ratio between 7x10^{-4} and 9x10^{-2} is derived in the outflows. In the warm inner regions of these two sources, we estimate the HDO/H2O ratios at about 1x10^{-4}-4x10^{-3}. This ratio seems higher (a few %) in the cold envelope of IRAS4A, whose possible origin is discussed in relation to formation processes of HDO and H2O.
    Full-text · Article · Oct 2013 · Astronomy and Astrophysics
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: [Context] Two competing models describe the formation of massive stars in objects like the Orion Trapezium. In the turbulent core accretion model, the resulting stellar masses are directly related to the mass distribution of the cloud condensations. In the competitive accretion model, the gravitational potential of the protocluster captures gas from the surrounding cloud for which the individual cluster members compete. [Aims] With high resolution submillimeter observations of the structure, kinematics, and chemistry of the proto-Trapezium cluster W3 IRS5, we aim to determine which mode of star formation dominates. [Methods] We present 354 GHz Submillimeter Array observations at resolutions of 1"-3" (1800-5400 AU) of W3 IRS5. ...... [Results] The observations show five emission peaks (SMM1-5). SMM1 and SMM2 contain massive embedded stars (~20 Msun); SMM3-5 are starless or contain low-mass stars (<8 Msun). The inferred densities are high, >= 10^7 cm^-3, but the core masses are small, 0.2-0.6 Msun. The detected molecular emission reveals four different chemical zones. ...... [Conclusions] The proto-Trapezium cluster W3 IRS5 is an ideal test case to discriminate between models of massive star formation. Either the massive stars accrete locally from their local cores; in this case the small core masses imply that W3 IRS5 is at the very end stages (1000 yr) of infall and accretion, or the stars are accreting from the global collapse of a massive, cluster forming core. We find that the observed masses, densities and line widths observed toward W3 IRS 5 and the surrounding cluster forming core are consistent with the competitive accretion of gas at rates of Macc~10^-4 Msun yr^-1 by the massive young forming stars. ......
    Full-text · Article · Aug 2013 · Astronomy and Astrophysics
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Context. Outflows are an important part of the star formation process as both the result of ongoing active accretion and one of the main sources of mechanical feedback on small scales. Water is the ideal tracer of these effects because it is present in high abundance for the conditions expected in various parts of the protostar, particularly the outflow. Aims. We constrain and quantify the physical conditions probed by water in the outflow-jet system for Class 0 and I sources. Methods. We present velocity-resolved Herschel HIFI spectra of multiple water-transitions observed towards 29 nearby Class 0/I protostars as part of the WISH guaranteed time key programme. The lines are decomposed into different Gaussian components, with each component related to one of three parts of the protostellar system; quiescent envelope, cavity shock and spot shocks in the jet and at the base of the outflow. We then use non-LTE radex models to constrain the excitation conditions present in the two outflow-related components. Results. Water emission at the source position is optically thick but effectively thin, with line ratios that do not vary with velocity, in contrast to CO. The physical conditions of the cavity and spot shocks are similar, with post-shock H-2 densities of order 10(5) -10(8) cm(-3) and H2O column densities of order 10(16) -10(18) cm(-2). H2O emission originates in compact emitting regions: for the spot shocks these correspond to point sources with radii of order 10-200 AU, while for the cavity shocks these come from a thin layer along the outflow cavity wall with thickness of order 1-30 AU. Conclusions. Water emission at the source position traces two distinct kinematic components in the outflow; J shocks at the base of the outflow or in the jet, and C shocks in a thin layer in the cavity wall. The similarity of the physical conditions is in contrast to off-source determinations which show similar densities but lower column densities and larger filling factors. We propose that this is due to the differences in shock properties and geometry between these positions. Class I sources have similar excitation conditions to Class 0 sources, but generally smaller line-widths and emitting region sizes. We suggest that it is the velocity of the wind driving the outflow, rather than the decrease in envelope density or mass, that is the cause of the decrease in H2O intensity between Class 0 and I sources.
    Preview · Article · Jun 2013 · Astronomy and Astrophysics
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Context. Our understanding of the star formation process has traditionally been confined to certain mass or luminosity boundaries because most studies focus only on low-, intermediate-, or high-mass star-forming regions. Therefore, the processes that regulate the formation of these different objects have not been effectively linked. As part of the "Water In Star-forming regions with Herschel" (WISH) key programme, water and other important molecules, such as CO and OH, have been observed in 51 embedded young stellar objects (YSOs). The studied sample covers a range of luminosities from <1 to >10(5) L-circle dot. Aims. We analyse the CO line emission towards a large sample of embedded protostars in terms of both line intensities and profiles. This analysis covers a wide luminosity range in order to achieve better understanding of star formation without imposing luminosity boundaries. In particular, this paper aims to constrain the dynamics of the environment in which YSOs form. Methods. Herschel-HIFI spectra of the (CO)-C-12 J = 10-9, (CO)-C-13 J = 10-9 and (CO)-O-18 J = 5-4, J = 9-8 and J = 10-9 lines were analysed for a sample of 51 embedded protostars. In addition, JCMT spectra of (CO)-C-12 J = 3-2 and (CO)-O-18 J = 3-2 extend this analysis to cooler gas components. We focussed on characterising the shape and intensity of the CO emission line profiles by fitting the lines with one or two Gaussian profiles. We compared the values and results of these fits across the entire luminosity range covered by WISH observations. The effects of different physical parameters as a function of luminosity and the dynamics of the envelope-outflow system were investigated. Results. All observed CO and isotopologue spectra show a strong linear correlation between the logarithms of the line and bolometric luminosities across six orders of magnitude on both axes. This suggests that the high-J CO lines primarily trace the amount of dense gas associated with YSOs and that this relation can be extended to larger (extragalactic) scales. The majority of the detected (CO)-C-12 line profiles can be decomposed into a broad and a narrow Gaussian component, while the (CO)-O-18 spectra are mainly fitted with a single Gaussian. For low-and intermediate-mass protostars, the width of the (CO)-O-18 J = 9-8 line is roughly twice that of the (CO)-O-18 J = 3-2 line, suggesting increased turbulence/infall in the warmer inner envelope. For high-mass protostars, the line widths are comparable for lower-and higher-J lines. A broadening of the line profile is also observed from pre-stellar cores to embedded protostars, which is due mostly to non-thermal motions (turbulence/infall). The widths of the broad (CO)-C-12 J = 3-2 and J = 10-9 velocity components correlate with those of the narrow (CO)-O-18 J = 9-8 profiles, suggesting that the entrained outflowing gas and envelope motions are related but independent of the mass of the protostar. These results indicate that physical processes in protostellar envelopes have similar characteristics across the studied luminosity range.
    Preview · Article · May 2013 · Astronomy and Astrophysics
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report on ALMA observations of continuum and molecular line emission with 0.4" resolution towards the high-mass star forming region G35.20-0.74 N. Two dense cores are detected in typical hot-core tracers, such as CH3CN, which reveal velocity gradients. In one of these cores, the velocity field can be fitted with an almost edge-on Keplerian disk rotating about a central mass of 18 Msun. This finding is consistent with the results of a recent study of the CO first overtone bandhead emission at 2.3mum towards G35.20-0.74 N. The disk radius and mass are >2500 au and 3 Msun. To reconcile the observed bolometric luminosity (3x10^4 Lsun) with the estimated stellar mass of 18 Msun, we propose that the latter is the total mass of a binary system.
    Preview · Article · Apr 2013 · Astronomy and Astrophysics

Publication Stats

2k Citations
340.51 Total Impact Points

Institutions

  • 2009-2015
    • University of Groningen
      • Kapteyn Astronomical Institute
      Groningen, Groningen, Netherlands
  • 2010
    • University of Cologne
      • II. Institute of Physics
      Köln, North Rhine-Westphalia, Germany
    • The University of Calgary
      • Department of Physics and Astronomy
      Calgary, Alberta, Canada
  • 2008
    • University of Toledo
      • Department of Physics and Astronomy
      Toledo, Ohio, United States
  • 2006-2008
    • Netherlands Institute for Space Research, Utrecht
      Utrecht, Utrecht, Netherlands
  • 2007
    • GGD Groningen
      Groningen, Groningen, Netherlands
  • 2002-2006
    • Max Planck Institute for Radio Astronomy
      Bonn, North Rhine-Westphalia, Germany