17 research outputs found
Gas signatures of Herbig Ae/Be disks probed with Herschel SPIRE spectroscopy
Herbig Ae/Be objects, like their lower mass counterparts T Tauri stars, are
seen to form a stable circumstellar disk which is initially gas-rich and could
ultimately form a planetary system. We present Herschel SPIRE 460-1540 GHz
spectra of five targets out of a sample of 13 young disk sources, showing line
detections mainly due to warm CO gas.Comment: to be published in proceedings of IAU symposium 299 (Victoria, BC,
Canada, June 2013
Hydrogen Fluoride in High-Mass Star-forming Regions
Hydrogen fluoride has been established to be an excellent tracer of molecular
hydrogen in diffuse clouds. In denser environments, however, the HF abundance
has been shown to be approximately two orders of magnitude lower. We present
Herschel/HIFI observations of HF J=1-0 toward two high-mass star formation
sites, NGC6334 I and AFGL 2591. In NGC6334 I the HF line is seen in absorption
in foreground clouds and the source itself, while in AFGL 2591 HF is partially
in emission. We find an HF abundance with respect to H2 of 1.5e-8 in the
diffuse foreground clouds, whereas in the denser parts of NGC6334 I, we derive
a lower limit on the HF abundance of 5e-10. Lower HF abundances in dense clouds
are most likely caused by freeze out of HF molecules onto dust grains in
high-density gas. In AFGL 2591, the view of the hot core is obstructed by
absorption in the massive outflow, in which HF is also very abundant 3.6e-8)
due to the desorption by sputtering. These observations provide further
evidence that the chemistry of interstellar fluorine is controlled by freeze
out onto gas grains.Comment: accepted in Ap
Evidence for the start of planet formation in a young circumstellar disk
The growth of dust grains in protoplanetary disks is a necessary first step
towards planet formation. This growth has been inferred via observations of
thermal dust emission towards mature protoplanetary systems (age >2 million
years) with masses that are, on average, similar to Neptune3. In contrast, the
majority of confirmed exoplanets are heavier than Neptune. Given that young
protoplanetary disks are more massive than their mature counterparts, this
suggests that planet formation starts early, but evidence for grain growth that
is spatially and temporally coincident with a massive reservoir in young disks
remains scarce. Here, we report observations on a lack of emission of carbon
monoxide isotopologues within the inner ~15 au of a very young (age ~100,000
years) disk around the Solar-type protostar TMC1A. By using the absence of
spatially resolved molecular line emission to infer the gas and dust content of
the disk, we conclude that shielding by millimeter-size grains is responsible
for the lack of emission. This suggests that grain growth and millimeter-size
dust grains can be spatially and temporally coincident with a mass reservoir
sufficient for giant planet formation. Hence, planet formation starts during
the earliest, embedded phases in the life of young stars.Comment: Accepted for publication in Nature Astronomy, 3 figures, 3 extended
figure
Episodic infall towards a compact disk in B335?
Previous observations of B335 have presented evidence of ongoing infall in
various molecular lines, e.g., HCO, HCN, CO. There have been no confirmed
observations of a rotationally supported disk on scales greater than ~12~au.
The presence of an outflow in B335 suggests that also a disk should be present
or in formation. To constrain the earliest stages of protostellar evolution and
disk formation, we aim to map the region where gas falls inwards and
observationally constrain its kinematics. Furthermore, we aim to put strong
limits on the size and orientation of any disk-like structure in B335. We use
high angular resolution CO data from ALMA, and combine it with
shorter-baseline archival data to produce a high-fidelity image of the infall
in B335. We also revisit the imaging of high-angular resolution Band 6
continuum data to study the dust distribution in the immediate vicinity of
B335. Continuum emission shows an elliptical structure (10 by 7 au) with a
position angle 5 degrees east of north, consistent with the expectation for a
forming disk in B335. A map of the infall velocity (as estimated from the
CO emission), shows evidence of asymmetric infall, predominantly from
the north and south. Close to the protostar, infall velocities appear to exceed
free-fall velocities. 3D radiative transfer models, where the infall velocity
is allowed to vary within the infall region, can explain the observed
kinematics. The data suggests that a disk has started to form in B335 and that
gas is falling towards that disk. However, kinematically-resolved line data
towards the disk itself is needed to confirm the presence of a rotationally
supported disk around this young protostar. The measured high infall velocities
are not easily reconcilable with a magnetic braking scenario and suggest that
there is a pressure gradient that allows the infall velocity to vary in the
region.Comment: 14 pages, 11 figure
Observing Extended Sources with the \Herschel SPIRE Fourier Transform Spectrometer
The Spectral and Photometric Imaging Receiver (SPIRE) on the European Space
Agency's Herschel Space Observatory utilizes a pioneering design for its
imaging spectrometer in the form of a Fourier Transform Spectrometer (FTS). The
standard FTS data reduction and calibration schemes are aimed at objects with
either a spatial extent much larger than the beam size or a source that can be
approximated as a point source within the beam. However, when sources are of
intermediate spatial extent, neither of these calibrations schemes is
appropriate and both the spatial response of the instrument and the source's
light profile must be taken into account and the coupling between them
explicitly derived. To that end, we derive the necessary corrections using an
observed spectrum of a fully extended source with the beam profile and the
source's light profile taken into account. We apply the derived correction to
several observations of planets and compare the corrected spectra with their
spectral models to study the beam coupling efficiency of the instrument in the
case of partially extended sources. We find that we can apply these correction
factors for sources with angular sizes up to \theta_{D} ~ 17". We demonstrate
how the angular size of an extended source can be estimated using the
difference between the sub-spectra observed at the overlap bandwidth of the two
frequency channels in the spectrometer, at 959<\nu<989 GHz. Using this
technique on an observation of Saturn, we estimate a size of 17.2", which is 3%
larger than its true size on the day of observation. Finally, we show the
results of the correction applied on observations of a nearby galaxy, M82, and
the compact core of a Galactic molecular cloud, Sgr B2.Comment: Accepted for publication by A&
Gas signatures of Herbig Ae/Be disks probed with Herschel SPIRE spectroscopy
Herbig Ae/Be objects, like their lower mass counterparts T Tauri stars, are seen to form a stable circumstellar disk which is initially gas-rich and could ultimately form a planetary system. We present Herschel SPIRE 460-1540 GHz spectra of five targets out of a sample of 13 young disk sources, showing line detections mainly due to warm CO gas