36 research outputs found
NGC7538 IRS1 - an ionized jet powered by accretion
Analysis of high spatial resolution VLA images shows that the free-free
emission from NGC7538 IRS1 is dominated by a collimated ionized wind. We have
re-analyzed high angular resolution VLA archive data from 6 cm to 7 mm, and
measured separately the flux density from the compact bipolar core and the
extended (1.5" - 3") lobes. We find that the flux density of the core is
proportional to the frequency to the power of alpha, with alpha being about
0.7. The frequency dependence of the total flux density is slightly steeper
with alpha = 0.8. A massive optically thick hypercompact core with a steep
density gradient can explain this frequency dependence, but it cannot explain
the extremely broad recombination line velocities observed in this source.
Neither can it explain why the core is bipolar rather than spherical, nor the
observed decrease of 4% in the flux density in less than 10 years. An ionized
wind modulated by accretion is expected to vary, because the accretion flow
from the surrounding cloud will vary over time. BIMA and CARMA continuum
observations at 3 mm show that the free-free emission still dominates at 3 mm.
HCO+ J = 1 - 0 observations combined with FCRAO single dish data show a clear
inverse P Cygni profile towards IRS1. These observations confirm that IRS1 is
heavily accreting with an accretion rate of about 2 times 10(-4) solar masses
per year.Comment: Accepted for Astrophysical Journal Letter
Observations of a high-mass protostar in NGC 7538S
We present high angular resolution continuum observations of the high-mass
protostar NGC 7538S with BIMA and CARMA at 3 and 1.4 mm, VLA observations at
1.3, 2, 3.5 and 6 cm, and archive IRAC observations from the Spitzer Space
Observatory, which detect the star at 4.5, 5.8, and 8 m. The star looks
rather unremarkable in the mid-IR. The excellent positional agreement of the
IRAC source with the VLA free-free emission, the OH, CHOH, HO masers,
and the dust continuum confirms that this is the most luminous object in the
NGC 7538S core. The continuum emission at millimeter wavelengths is dominated
by dust emission from the dense cold cloud core surrounding the protostar.
Including all array configurations, the emission is dominated by an elliptical
source with a size of ~ 8" x 3". If we filter out the extended emission we find
three compact mm-sources inside the elliptical core. The strongest one, ,
coincides with the VLA/IRAC source and resolves into a double source at 1.4 mm,
where we have sub-arcsecond resolution. The measured spectral index, ,
between 3 and 1.4 mm is ~ 2.3, and steeper at longer wavelengths, suggesting a
low dust emissivity or that the dust is optically thick. We argue that the dust
in these accretion disks is optically thick and estimate a mass of an accretion
disk or infalling envelope surrounding S to be ~ 60 solar masses.Comment: ApJ Accepted 2012, 13 pages, 9 figure
A molecular line study of NGC 1333/IRAS 4
Molecular line surveys and fully sampled spectral line maps at 1.3 and 0.87 mm are used to examine the
physical and chemical characteristics of the extreme Class I sources IRAS 4A and 4B in the L1450/NGC 1333
molecular cloud complex. A very well collimated, jetlike molecular outflow emanates from IRAS 4A, with a
dynamical age of a few thousand years. Symmetric, clumpy structure along the outflow lobes suggests that
there is considerable variability in the mass-loss rate or wind velocity even at this young age. Molecular emission
lines toward IRAS 4A and 4B are observed to be weak in the velocity range corresponding to quiescent
material surrounding the young stellar objects (YSOs). Depletion factors of 10-20 are observed for αll molecules,
including CO, even for very conservative mass estimates from the measured millimeter and submillimeter
dust continuum. However, abundances scaled with respect to CO are similar to other dark
molecular cloud cores. Such depletions could be mimicked by high dust optical depths or increased grain
emissivities at the observing frequencies of 230 and 345 GHz, but the millimeter and submillimeter spectral
energy distributions suggest that this is unlikely over the single-dish size scales of 5000-10,000 AU.
Dense, outflowing gas is found to be kinematically, but not spatially, distinct from the quiescent material on
these size scales. If CO is used as a chemical standard for the high-velocity gas, we find substantial enhancements
in the abundances of several molecules in outflowing material, most notably CS, SiO, and CH_30H. The
SiO emission is kinematically well displaced from the bulk cloud velocity and likely arises from directly
shocked material. As is the case for CO, however, the outflow features from more volatile species are centered
near the cloud velocity and are often characterized by quite low rotational temperatures. We suggest that
grain-grain collisions induced by velocity shear zones surrounding the outflow axes transiently desorb the
grain mantles, resulting in large abundance enhancements of selected species. Similar results have recently
been obtained in several other low-mass YSOs, where the outflowing gas is often both kinematically and spatially
distinct, and are illustrative of the ability of accretion and outflow processes to simultaneously modify
the composition of the gas and dust surrounding young stars
A detailed study of the accretion disk surrounding the high-mass protostar NGC 7538S
We present deep high angular resolution observations of the high-mass
protostar NGC 7538S, which is in the center of a cold dense cloud core with a
radius of 0.5 pc and a mass of ~2,000 Msun. These observations show that NGC
7538S is embedded in a compact elliptical core with a mass of 85 - 115 Msun.
The star is surrounded by a rotating accretion disk, which powers a very young,
hot molecular outflow approximately perpendicular to the rotating accretion
disk. The accretion rate is very high, ~ 1.4 - 2.8 10^-3 Msun yr^-1. Evidence
for rotation of the disk surrounding the star is seen in all largely optically
thin molecular tracers, H13CN J = 1-0, HN13C J = 1-0, H13CO+ J = 1-0, and DCN J
= 3-2. Many molecules appear to be affected by the hot molecular outflow,
including DCN and H13CO+. The emission from CH3CN, which has often been used to
trace disk rotation in young high-mass stars, is dominated by the outflow,
especially at higher K-levels. Our new high-angular resolution observations
show that the rotationally supported part of the disk is smaller than we
previously estimated. The enclosed mass of the inner, rotationally supported
part of the disk (D ~ 5", i.e 14,000 AU) is ~ 14 - 24 Msun.Comment: Accepted by ApJ; 20 pages, 20 figure
Sub-arcsec Observations of NGC 7538 IRS 1: Continuum Distribution and Dynamics of Molecular Gas
We report new results based on the analysis of the SMA and CARMA observations
of NGC 7538\,IRS\,1 at 1.3 and 3.4 mm with sub-arcsec resolutions. With angular
resolutions 0\farcs7, the SMA and CARMA observations show that the
continuum emission at 1.3 and 3.4 mm from the hyper-compact \ion{H}{2} region
IRS\,1 is dominated by a compact source with a tail-like extended structure to
the southwest of IRS\,1. With a CARMA B-array image at 1.3 mm convolved to
0\farcs1, we resolve the hyper-compact \ion{H}{2} region into two components:
an unresolved hyper-compact core, and a north-south extension with linear sizes
of AU and 2000 AU, respectively. The fine structure observed with
CARMA is in good agreement with the previous VLA results at centimeter
wavelengths, suggesting that the hyper-compact \ion{H}{2} region at the center
of IRS\,1 is associated with an ionized bipolar outflow. We image the molecular
lines OCS(19-18) and CHCN(12-11) as well as CO(2-1) surrounding
IRS\,1, showing a velocity gradient along the southwest-northeast direction.
The spectral line profiles in CO(2-1), CO(2-1), and HCN(1-0) observed
toward IRS\,1 show broad redshifted absorption, providing evidence for gas
infall with rates in the range of M yr
inferred from our observations.Comment: 19 pages, 14 figure
A major radio outburst in III Zw 2 with an extremely inverted, millimeter-peaked spectrum
III Zw 2 is a spiral galaxy with an optical spectrum and faint extended radio
structure typical of a Seyfert galaxy, but also with an extremely variable,
blazar-like radio core. We have now discovered a new radio flare where the
source has brightened more than twenty-fold within less than two years. A
broad-band radio spectrum between 1.4 and 666 GHz shows a textbook-like
synchrotron spectrum peaking at 43 GHz, with a self-absorbed synchrotron
spectral index +2.5 at frequencies below 43 GHz and an optically thin spectral
index -0.75 at frequencies above 43 GHz. The outburst spectrum can be well
fitted by two homogenous, spherical components with equipartition sizes of 0.1
and 0.2 pc at 43 and 15 GHz, and with magnetic fields of 0.4 and 1 Gauss. VLBA
observations at 43 GHz confirm this double structure and these sizes. Time
scale arguments suggest that the emitting regions are shocks which are
continuously accelerating particles. This could be explained by a frustrated
jet scenario with very compact hotspots. Similar millimeter-peaked spectrum
(MPS) sources could have escaped our attention because of their low flux
density at typical survey frequencies and their strong variability.Comment: ApJ Letters, in press, (AAS)LaTeX, 3 figures, available at
http://www2.mpifr-bonn.mpg.de/staff/hfalcke/publications.html#iiizw2 or in a
few weeks at
http://www.mpifr-bonn.mpg.de/staff/falcke/publications.html#iiizw
TADPOL: A 1.3 mm Survey of Dust Polarization in Star-forming Cores and Regions
We present {\lambda}1.3 mm CARMA observations of dust polarization toward 30
star-forming cores and 8 star-forming regions from the TADPOL survey. We show
maps of all sources, and compare the ~2.5" resolution TADPOL maps with ~20"
resolution polarization maps from single-dish submillimeter telescopes. Here we
do not attempt to interpret the detailed B-field morphology of each object.
Rather, we use average B-field orientations to derive conclusions in a
statistical sense from the ensemble of sources, bearing in mind that these
average orientations can be quite uncertain. We discuss three main findings:
(1) A subset of the sources have consistent magnetic field (B-field)
orientations between large (~20") and small (~2.5") scales. Those same sources
also tend to have higher fractional polarizations than the sources with
inconsistent large-to-small-scale fields. We interpret this to mean that in at
least some cases B-fields play a role in regulating the infall of material all
the way down to the ~1000 AU scales of protostellar envelopes. (2) Outflows
appear to be randomly aligned with B-fields; although, in sources with low
polarization fractions there is a hint that outflows are preferentially
perpendicular to small-scale B-fields, which suggests that in these sources the
fields have been wrapped up by envelope rotation. (3) Finally, even at ~2.5"
resolution we see the so-called "polarization hole" effect, where the
fractional polarization drops significantly near the total intensity peak. All
data are publicly available in the electronic edition of this article.Comment: 53 pages, 37 figures -- main body (13 pp., 3 figures), source maps
(32 pp., 34 figures), source descriptions (8 pp.). Accepted by the
Astrophysical Journal Supplemen
HD 172555: Detection of 63 Ό m [OI] emission in a debris disc
Astronomy and Astrophysics 546 (2012): L8 Reproduced with permission from Astronomy & AstrophysicsContext. HD 172555 is a young A7 star belonging to the ÎČ Pictoris moving group that harbours a debris disc. The Spitzer/IRS spectrum of the source showed mid-IR features such as silicates and glassy silica species, indicating the presence of a warm dust component with small grains, which places HD 172555 among the small group of debris discs with such properties. The IRS spectrum also shows a possible emission of SiO gas.
Aims. We aim to study the dust distribution in the circumstellar disc of HD 172555 and to asses the presence of gas in the debris disc.
Methods. As part of the GASPS open time key programme, we obtained Herschel/PACS photometric and spectroscopic observations of the source.We analysed PACS observations of HD 172555 and modelled the spectral energy distribution with a modified blackbody and the gas emission with a two-level population model with no collisional de-excitation.
Results. We report for the first time the detection of [OI] atomic gas emission at 63.18 ÎŒm in the HD 172555 circumstellar disc. We detect excesses due to circumstellar dust toward HD 172555 in the three photometric bands of PACS (70, 100, and 160 ÎŒm).We derive a large dust particle mass of (4.8 ± 0.6) Ă 10â4 Mâ and an atomic oxygen mass of 2.5 Ă 10â2R2 Mâ, where R in AU is the separation between the star and the inner disc. Thus, most of the detected mass of the disc is in the gaseous phaseThis research has been funded by Spanish grants AYA 2010-21161-C02-02, CDS2006-00070 and PRICIT-S2009/ESP-1496. J.-C. Augereau and J. Lebreton thank the ANR (contract ANR-2010 BLAN-0505-01, EXOZODI) and the CNES-PNP for financial support. C. Pinte, F. Menard and W.-F. Thi acknowledges funding from the EU FP7-2011 under Grant Agreement nr. 284405. G. Meeus is supported by RYC-2011-07920. G. Meeus, C. Eiroa, I. MendigutĂa and B. Montesinos are partly supported by AYA-2011-26202. F.M. acknowledges support from the Millennium Science Initiative (Chilean Ministry of Economy), through grant ĂNucleus P10-022-F
Near-Infrared Spectroscopy of TW Hya: A Revised Spectral Type and Comparison with Magnetospheric Accretion Models
We present high signal-to-noise, moderate spectral resolution (R ~ 2000-2500)
near-infrared (0.8-5.0 micron) spectroscopy of the nearby T Tauri star TW Hya.
By comparing the spectrum and the equivalent widths of several atomic and
molecular features with those for stars in the IRTF near-infrared library, we
revise the spectral type to M2.5V, which is later than usually adopted (K7V).
This implies a substantially cooler stellar temperature than previously
assumed. Comparison with various pre-main sequence models suggests that TW Hya
is only ~3 Myr old; much younger than the usually adopted 8 - 10 Myr. Analysis
of the relative strengths of the H lines seen in the spectrum yields estimates
for the temperature and density of the emitting region of T_e > 7500 K and n_e
~ 10^{12} - 10^{13} cm^{-3}. The thickness of the emitting region is 10^2 -
10^4 km and the covering fraction is f_\ast ~ 0.04. Our derived physical
parameter values agree with the predictions of the magnetospheric accretion
scenario. The highest signal-to-noise H lines have profiles that indicate
multiple emission components. We derive an excess spectrum (above that of the
M2.5V template) that peaks in the H band. Although our derived veiling values,
~ 0.1, agree with previous estimates, the excess spectrum does not match that
of current models in which this flux is generated by an inner optically thin
disk. We suggest that the excess flux spectrum instead reflects the differences
in atmospheric opacity, gravity, and age between TW Hya and older, higher
gravity field M2.5 dwarfs.Comment: 43 pages, 12 figures; accepted for publication in Ap