1,317 research outputs found
Disk masses in the Orion Molecular Cloud-2: distinguishing time and environment
The mass evolution of protoplanetary disks is driven by both internal
processes and external factors, such as photoevaporation. Disentangling these
two effects, however, has remained difficult. We measure the dust masses of a
sample of 132 disks in the Orion Molecular Cloud (OMC)-2 region, and compare
them to (i) externally photoevaporated disks in the Trapezium cluster, and (ii)
disks in nearby low-mass star forming regions (SFRs). This allows us to test if
initial disk properties are the same in high- and low-mass SFRs, and enables a
direct measurement of the effect of external photoevaporation on disks. A ~
mosaic of 3 mm continuum observations from the Atacama Large
Millimeter/submillimeter Array (ALMA) was used to measure the fluxes of 132
disks and 35 protostars >0.5 pc away from the Trapezium. We identify and
characterize a sample of 34 point sources not included in the Spitzer catalog
on which the sample is based. Of the disks, 37 (28%) are detected, with masses
ranging from 7-270 M_e. The detection rate for protostars is higher at 69%.
Disks near the Trapezium are found to be less massive by a factor
, implying a mass loss rate of M_sun/yr.
Our observations allow us to distinguish the impact of time and environment on
disk evolution in a single SFR. The disk mass distribution in OMC-2 is
statistically indistinguishable from that in nearby low-mass SFRs, like Lupus
and Taurus. We conclude that age is the main factor determining the evolution
of these disks. This result is robust with respect to assumptions of dust
temperature, sample incompleteness and biases. The difference between the OMC-2
and Trapezium cluster samples is consistent with mass loss driven by
far-ultraviolet radiation near the Trapezium. Together, this implies that in
isolation, disk formation and evolution proceed similarly, regardless of cloud
mass.Comment: Accepted for publication in A&A. 16 pages, 6 figure
Search for methylamine in high mass hot cores
We aim to detect methylamine, CHNH, in a variety of hot cores and
use it as a test for the importance of photon-induced chemistry in ice mantles
and mobility of radicals. Specifically, CHNH cannot be formed from atom
addition to CO whereas other NH-containing molecules such as formamide,
NHCHO, can. Submillimeter spectra of several massive hot core regions were
taken with the James Clerk Maxwell Telescope. Abundances are determined with
the rotational diagram method where possible. Methylamine is not detected,
giving upper limit column densities between 1.9 6.4 10
cm for source sizes corresponding to the 100 K envelope radius. Combined
with previously obtained JCMT data analyzed in the same way, abundance ratios
of CHNH, NHCHO and CHCN with respect to each other and
to CHOH are determined. These ratios are compared with Sagittarius B2
observations, where all species are detected, and to hot core models. The
observed ratios suggest that both methylamine and formamide are overproduced by
up to an order of magnitude in hot core models. Acetonitrile is however
underproduced. The proposed chemical schemes leading to these molecules are
discussed and reactions that need further laboratory studies are identified.
The upper limits obtained in this paper can be used to guide future
observations, especially with ALMA.Comment: 14 pages, 5 figures, accepted for publication in A&
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
Photodissociation and photoionisation of atoms and molecules of astrophysical interest
A new collection of photodissociation and photoionisation cross sections for
102 atoms and molecules of astrochemical interest has been assembled, along
with a brief review of the basic processes involved. These have been used to
calculate dissociation and ionisation rates, with uncertainties, in a standard
ultraviolet interstellar radiation field (ISRF) and wavelength-dependent
radiation fields. The new ISRF rates generally agree within 30% with our
previous compilations, with a few notable exceptions. The reduction of rates in
shielded regions was calculated as a function of dust, molecular and atomic
hydrogen, atomic C, and self-shielding column densities. The relative
importance of shielding types depends on the species in question and the dust
optical properties. The new data are publicly available from the Leiden
photodissociation and ionisation database.
Sensitivity of rates to variation of temperature and isotope, and cross
section uncertainties, are tested. Tests were conducted with an
interstellar-cloud chemical model, and find general agreement (within a factor
of two) with the previous iteration of the Leiden database for the ISRF, and
order-of-magnitude variations assuming various kinds of stellar radiation. The
newly parameterised dust-shielding factors makes a factor-of-two difference to
many atomic and molecular abundances relative to parameters currently in the
UDfA and KIDA astrochemical reaction databases. The newly-calculated cosmic-ray
induced photodissociation and ionisation rates differ from current standard
values up to a factor of 5. Under high temperature and cosmic-ray-flux
conditions the new rates alter the equilibrium abundances of abundant dark
cloud abundances by up to a factor of two. The partial cross sections for H2O
and NH3 photodissociation forming OH, O, NH2 and NH are also evaluated and lead
to radiation-field-dependent branching ratios.Comment: Corrected some inconsistent table/figure data. Significant change: Zn
photoionisation rate corrected. Accepted for publication by A&
Astrochemical confirmation of the rapid evolution of massive YSOs and explanation for the inferred ages of hot cores
Aims. To understand the roles of infall and protostellar evolution on the
envelopes of massive young stellar objects (YSOs).
Methods. The chemical evolution of gas and dust is traced, including infall
and realistic source evolution. The temperatures are determined
self-consistently. Both ad/desorption of ices using recent laboratory
temperature-programmed-desorption measurements are included.
Results. The observed water abundance jump near 100 K is reproduced by an
evaporation front which moves outward as the luminosity increases. Ion-molecule
reactions produce water below 100 K. The age of the source is constrained to t
\~ 8 +/- 4 x 10^4 yrs since YSO formation. It is shown that the chemical
age-dating of hot cores at ~ few x 10^3 - 10^4 yr and the disappearance of hot
cores on a timescale of ~ 10^5 yr is a natural consequence of infall in a
dynamic envelope and protostellar evolution. Dynamical structures of ~ 350AU
such as disks should contain most of the complex second generation species. The
assumed order of desorption kinetics does not affect these results.Comment: Accepted by A&A Letters; 4 pages, 5 figure
Dense and warm molecular gas in the envelopes and outflows of southern low-mass protostars
Observations of dense molecular gas lie at the basis of our understanding of
the density and temperature structure of protostellar envelopes and molecular
outflows. We aim to characterize the properties of the protostellar envelope,
molecular outflow and surrounding cloud, through observations of high
excitation molecular lines within a sample of 16 southern sources presumed to
be embedded YSOs. Observations of submillimeter lines of CO, HCO+ and their
isotopologues, both single spectra and small maps were taken with the FLASH and
APEX-2a instruments mounted on APEX to trace the gas around the sources. The
HARP-B instrument on the JCMT was used to map IRAS 15398-3359 in these lines.
HCO+ mapping probes the presence of dense centrally condensed gas, a
characteristic of protostellar envelopes. The rare isotopologues C18O and
H13CO+ are also included to determine the optical depth, column density, and
source velocity. The combination of multiple CO transitions, such as 3-2, 4-3
and 7-6, allows to constrain outflow properties, in particular the temperature.
Archival submillimeter continuum data are used to determine envelope masses.
Eleven of the sixteen sources have associated warm and/or dense quiescent as
characteristic of protostellar envelopes, or an associated outflow. Using the
strength and degree of concentration of the HCO+ 4-3 and CO 4-3 lines as a
diagnostic, five sources classified as Class I based on their spectral energy
distributions are found not to be embedded YSOs. The C18O 3-2 lines show that
for none of the sources, foreground cloud layers are present. Strong molecular
outflows are found around six sources, .. (continued in paper)Comment: Accepted by A&A, 13 figure
The deuterium fractionation of water on solar-system scales in deeply-embedded low-mass protostars
(Abridged) The water deuterium fractionation (HDO/HO abundance ratio) has
traditionally been used to infer the amount of water brought to Earth by
comets. Measuring this ratio in deeply-embedded low-mass protostars makes it
possible to probe the critical stage when water is transported from clouds to
disks in which icy bodies are formed. We present sub-arcsecond resolution
observations of HDO in combination with HO from the PdBI toward the
three low-mass protostars NGC 1333-IRAS 2A, IRAS 4A-NW, and IRAS 4B. The
resulting HDO/HO ratio is for IRAS 2A,
for IRAS 4A-NW, and for IRAS
4B. Derived ratios agree with radiative transfer models within a factor of 2-4
depending on the source. Our HDO/HO ratios for the inner regions (where
K) of four young protostars are only a factor of 2 higher than those
found for pristine, solar system comets. These small differences suggest that
little processing of water occurs between the deeply embedded stage and the
formation of planetesimals and comets.Comment: 10 pages, 6 figures, accepted for publication in Astronomy and
Astrophysic
Detection of interstellar H_2D^+ emission
We report the detection of the 1_{10}-1_{11} ground state transition of
ortho-H_2D^+ at 372.421 GHz in emission from the young stellar object NGC 1333
IRAS 4A. Detailed excitation models with a power-law temperature and density
structure yield a beam-averaged H_2D^+ abundance of 3 x 10^{-12} with an
uncertainty of a factor of two. The line was not detected toward W 33A, GL
2591, and NGC 2264 IRS, in the latter source at a level which is 3-8 times
lower than previous observations. The H_2D^+ data provide direct evidence in
support of low-temperature chemical models in which H_2D^+ is enhanced by the
reaction of H_3^+ and HD. The H_2D^+ enhancement toward NGC 1333 IRAS 4A is
also reflected in the high DCO^+/HCO^+ abundance ratio. Simultaneous
observations of the N_2H^+ 4-3 line show that its abundance is about 50-100
times lower in NGC 1333 IRAS 4A than in the other sources, suggesting
significant depletion of N_2. The N_2H^+ data provide independent lower limits
on the H_3^+ abundance which are consistent with the abundances derived from
H_2D^+. The corresponding limits on the H_3^+$ column density agree with recent
near-infrared absorption measurements of H_3^+ toward W 33A and GL 2591.Comment: Standard AAS LaTeX format (15 pages + 2 figures
CN rings in full protoplanetary disks around young stars as probes of disk structure
Bright ring-like structure emission of the CN molecule has been observed in
protoplanetary disks. We investigate whether such structures are due to the
morphology of the disk itself or if they are instead an intrinsic feature of CN
emission. With the intention of using CN as a diagnostic, we also address to
which physical and chemical parameters CN is most sensitive. A set of disk
models were run for different stellar spectra, masses, and physical structures
via the 2D thermochemical code DALI. An updated chemical network that accounts
for the most relevant CN reactions was adopted. Ring-shaped emission is found
to be a common feature of all adopted models; the highest abundance is found in
the upper outer regions of the disk, and the column density peaks at 30-100 AU
for T Tauri stars with standard accretion rates. Higher mass disks generally
show brighter CN. Higher UV fields, such as those appropriate for T Tauri stars
with high accretion rates or for Herbig Ae stars or for higher disk flaring,
generally result in brighter and larger rings. These trends are due to the main
formation paths of CN, which all start with vibrationally excited H2*
molecules, that are produced through far ultraviolet (FUV) pumping of H2. The
model results compare well with observed disk-integrated CN fluxes and the
observed location of the CN ring for the TW Hya disk. CN rings are produced
naturally in protoplanetary disks and do not require a specific underlying disk
structure such as a dust cavity or gap. The strong link between FUV flux and CN
emission can provide critical information regarding the vertical structure of
the disk and the distribution of dust grains which affects the UV penetration,
and could help to break some degeneracies in the SED fitting. In contrast with
C2H or c-C3H2, the CN flux is not very sensitive to carbon and oxygen
depletion.Comment: New version of paper, correcting too high H2 excitation rates and
consequently too high CN column densities. Qualitative conclusions of the
paper remain unchanged. Quantitatively, the CN column densities are an order
of magnitude lower whereas fluxes decrease by a factor of 3-4. Rings are
larger by up to a factor of 2. 13 pages, 19 figures, accepted for publication
in A&
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