559 research outputs found
Energetic radiation and the sulfur chemistry of protostellar envelopes: Submillimeter interferometry of AFGL 2591
CONTEXT: The chemistry in the inner few thousand AU of accreting envelopes
around young stellar objects is predicted to vary greatly with far-UV and X-ray
irradiation by the central star. Aim We search for molecular tracers of
high-energy irradiation by the protostar in the hot inner envelope. METHODS:
The Submillimeter Array (SMA) has observed the high-mass star forming region
AFGL 2591 in lines of CS, SO, HCN, HCN(v2=1), and HC15N with 0.6" resolution at
350 GHz probing radial scales of 600-3500 AU for an assumed distance of 1 kpc.
The SMA observations are compared with the predictions of a chemical model
fitted to previous single-dish observations. RESULTS: The CS and SO main peaks
are extended in space at the FWHM level, as predicted in the model assuming
protostellar X-rays. However, the main peak sizes are found smaller than
modeled by nearly a factor of 2. On the other hand, the lines of CS, HCN, and
HC15N, but not SO and HCN(v2=1), show pedestal emissions at radii of about 3500
AU that are not predicted. All lines except SO show a secondary peak within the
approaching outflow cone. A dip or null in the visibilities caused by a sharp
decrease in abundance with increasing radius is not observed in CS and only
tentatively in SO. CONCLUSIONS: The emission of protostellar X-rays is
supported by the good fit of the modeled SO and CS amplitude visibilities
including an extended main peak in CS. The broad pedestals can be interpreted
by far-UV irradiation in a spherically non-symmetric geometry, possibly
comprising outflow walls on scales of 3500 -- 7000 AU. The extended CS and SO
main peaks suggest sulfur evaporation near the 100 K temperature radius.Comment: Astronomy and Astrophysics, in pres
Very compact radio emission from high-mass protostars. II. Dust disks and ionized accretion flows
This paper reports 43 GHz imaging of the high-mass protostars W33A, AFGL 2591
and NGC 7538 IRS9 at 0.04'' and 0.6'' resolution. In each case, weak (~mJy),
compact (~100 AU) emission is detected, which has an elongated shape (axis
ratio ~3). For AFGL 2591 and NGC 7538 IRS9, the emission is single-peaked,
while for the highest luminosity source, W33A, a `mini-cluster' of three
sources is detected. The derived sizes, flux densities, and broad-band radio
spectra of the sources support recent models where the initial expansion of HII
regions around very young O-type stars is prevented by stellar gravity. In
these models, accretion flows onto high-mass stars originate in large-scale
molecular envelopes and become ionized close to the star. These models
reproduce our observations of ionized gas as well as the structure of the
molecular envelopes of these sources on 10^3--10^4 AU scales derived previously
from single-dish submillimeter continuum and line mapping. For AFGL 2591, the
43 GHz flux density is also consistent with dust emission from a disk seen in
near-infrared `speckle' images. However, the alignment of the 43 GHz emission
with the large-scale molecular outflow argues against an origin in a disk for
AFGL 2591 and NGC 7538 IRS9. In contrast, the outflow from W33A does not appear
to be collimated. Together with previously presented case studies of W3 IRS5
and AFGL 2136, our results indicate that the formation of stars and stellar
clusters with luminosities up to ~10^5 L0 proceeds through accretion and
produces collimated outflows as in the solar-type case, with the `additional
feature' that the accretion flow becomes ionized close to the star. Above ~10^5
L0, clusters of HII regions appear, and outflows are no longer collimated,
possibly as the result of mergers of protostars or pre-stellar cores.Comment: Accepted by A&A; 11 pages, 4 b/w figure
The HIFI spectral survey of AFGL 2591 (CHESS). II. Summary of the survey
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.Comment: Accepted to A&
Kinematics of the inner thousand AU region around the young massive star AFGL 2591-VLA3: a massive disk candidate?
[Context] Recent detections of disks around young high-mass stars support the
idea of massive star formation through accretion rather than coalescence, but
the detailed kinematics in the equatorial region of the disk candidates is not
well known, which limits our understanding of the accretion process. [Aims]
This paper explores the kinematics of the gas around a young massive star with
millimeter-wave interferometry to improve our understanding of the formation of
massive stars though accretion. [Methods] We use Plateau de Bure
interferometric images to probe the environment of the nearby (~1 kpc) and
luminous (~20000 Lsun) high-mass (10-16 Msun) young star AFGL 2591-VLA3 in
continuum and in lines of HDO, H218O and SO2 in the 115 and 230 GHz bands.
Radiative transfer calculations are employed to investigate the kinematics of
the source. [Results] At ~0.5" (500 AU) resolution, the line images clearly
resolve the velocity field of the central compact source (diameter of ~ 800 AU)
and show linear velocity gradients in the northeast-southwest direction.
Judging from the disk-outflow geometry, the observed velocity gradient results
from rotation and radial expansion in the equatorial region of VLA3. Radiative
transfer calculations suggest that the velocity field is consistent with
sub-Keplerian rotation plus Hubble-law like expansion. The line profiles of the
observed molecules suggest a layered structure, with HDO emission arising from
the disk mid-plane, H218O from the warm mid-layer, and SO2 from the upper disk.
[Conclusions] We propose AFGL 2591-VLA3 as a new massive disk candidate, with
peculiar kinematics. The rotation of this disk is sub-Keplerian, probably due
to magnetic braking, while the stellar wind may be responsible for the
expansion of the disk. The expansion motion [...]Comment: 13 pages, 14 figures and 7 tables. accepted to A&
The subsurface habitability of small, icy exomoons
Context. Assuming our Solar System as typical, exomoons may outnumber exoplanets. If their habitability fraction is similar, they would thus constitute the largest portion of habitable real estate in the Universe. Icy moons in our Solar System, such as Europa and Enceladus, have already been shown to possess liquid water, a prerequisite for life on Earth. Aims: We intend to investigate under what thermal and orbital circumstances small, icy moons may sustain subsurface oceans and thus be "subsurface habitable". We pay specific attention to tidal heating, which may keep a moon liquid far beyond the conservative habitable zone. Methods: We made use of a phenomenological approach to tidal heating. We computed the orbit averaged flux from both stellar and planetary (both thermal and reflected stellar) illumination. We then calculated subsurface temperatures depending on illumination and thermal conduction to the surface through the ice shell and an insulating layer of regolith. We adopted a conduction only model, ignoring volcanism and ice shell convection as an outlet for internal heat. In doing so, we determined at which depth, if any, ice melts and a subsurface ocean forms. Results: We find an analytical expression between the moon's physical and orbital characteristics and the melting depth. Since this expression directly relates icy moon observables to the melting depth, it allows us to swiftly put an upper limit on the melting depth for any given moon. We reproduce the existence of Enceladus' subsurface ocean; we also find that the two largest moons of Uranus (Titania and Oberon) could well sustain them. Our model predicts that Rhea does not have liquid water. Conclusions: Habitable exomoon environments may be found across an exoplanetary system, largely irrespective of the distance to the host star. Small, icy subsurface habitable moons may exist anywhere beyond the snow line. This may, in future observations, expand the search area for extraterrestrial habitable environments beyond the circumstellar habitable zone
Water destruction by X-rays in young stellar objects
We study the H2O chemistry in star-forming environments under the influence
of a central X-ray source and a central far ultraviolet (FUV) radiation field.
The gas-phase water chemistry is modeled as a function of time, hydrogen
density and X-ray flux. To cover a wide range of physical environments,
densities between n_H = 10^4-10^9 cm^-3 and temperatures between T = 10-1000 K
are studied. Three different regimes are found: For T < 100 K, the water
abundance is of order 10^-7-10^-6 and can be somewhat enhanced or reduced due
to X-rays, depending on time and density. For 100 K < T < 250 K, H2O is reduced
from initial x(H2O) ~ 10^-4 following ice evaporation to x(H2O) ~ 10^-6 for F_X
> 10^-3 ergs s-1 cm^-2 (t = 10^4 yrs) and for F_X > 10^-4 ergs s^-1 cm^-2 (t =
10^5 yrs). At higher temperatures (T > 250 K) and hydrogen densities, water can
persist with x(H2O) ~ 10^-4 even for high X-ray fluxes. The X-ray and FUV
models are applied to envelopes around low-mass Class 0 and I young stellar
objects (YSOs). Water is destroyed in both Class 0 and I envelopes on
relatively short timescales (t ~ 5000 yrs) for realistic X-ray fluxes, although
the effect is less prominent in Class 0 envelopes due to the higher X-ray
absorbing densities there. FUV photons from the central source are not
effective in destroying water. The average water abundance in Class I sources
for L_X > 10^27 ergs s^-1 is predicted to be x(H2O) < 10^-6.Comment: 12 pages, 14 figures, Accepted for publication in A&
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
Enrichment of the HR 8799 planets by minor bodies and dust
Context. In the Solar System, minor bodies and dust deliver various materials to planetary surfaces. Several exoplanetary systems are known to host inner and outer belts, analogues of the main asteroid belt and the Kuiper belt, respectively. Aims: We study the possibility that exominor bodies and exodust deliver volatiles and refractories to the exoplanets in the well-characterised system HR 8799. Methods: We performed N-body simulations to study the impact rates of minor bodies in the system HR 8799. The model consists of the host star, four giant planets (HR 8799 e, d, c, and b), 650 000 test particles representing the inner belt, and 1 450 000 test particles representing the outer belt. Moreover we modelled dust populations that originate from both belts. Results: Within a million years, the two belts evolve towards the expected dynamical structure (also derived in other works), where mean-motion resonances with the planets carve the analogues of Kirkwood gaps. We find that, after this point, the planets suffer impacts by objects from the inner and outer belt at rates that are essentially constant with time, while dust populations do not contribute significantly to the delivery process. We convert the impact rates to volatile and refractory delivery rates using our best estimates of the total mass contained in the belts and their volatile and refractory content. Over their lifetime, the four giant planets receive between 10-4 and 10-3 M⊕ of material from both belts. Conclusions: The total amount of delivered volatiles and refractories, 5 × 10-3 M⊕, is small compared to the total mass of the planets, 11 × 103 M⊕. However, if the planets were formed to be volatile-rich, their exogenous enrichment in refractory material may well be significant and observable, for example with JWST-MIRI. If terrestrial planets exist within the snow line of the system, volatile delivery would be an important astrobiological mechanism and may be observable as atmospheric trace gases
Enrichment of the HR 8799 planets by minor bodies and dust
In the Solar System, minor bodies and dust deliver various materials to
planetary surfaces. Several exoplanetary systems are known to host inner and
outer belts, analogues of the main asteroid belt and the Kuiper belt. We study
the possibility that exominor bodies and exodust deliver volatiles and
refractories to the exoplanets in the system HR8799 by performing N-body
simulations. The model consists of the host star, four giant planets (HR8799 e,
d, c, and b), 650000 test particles representing the inner belt, and 1450000
test particles representing the outer belt. Moreover we modelled dust
populations that originate from both belts. Within a million years, the two
belts evolve towards the expected dynamical structure (also derived in other
works), where mean-motion resonances with the planets carve the analogues of
Kirkwood gaps. We find that, after this point, the planets suffer impacts by
objects from the inner and outer belt at rates that are essentially constant
with time, while dust populations do not contribute significantly to the
delivery process. We convert the impact rates to volatile and refractory
delivery rates using our best estimates of the total mass contained in the
belts and their volatile and refractory content. Over their lifetime, the four
giant planets receive between and 10^{-3}M_\bigoplus of material
from both belts. The total amount of delivered volatiles and refractories,
{5\times10^{-3}\textrm{M}_\bigoplus}, is small compared to the total mass of
the planets, 11\times10^{3}\textrm{M}_\bigoplus. However, if the planets were
formed to be volatile-rich, their exogenous enrichment in refractory material
may well be significant and observable, for example with JWST-MIRI. If
terrestrial planets exist within the snow line of the system, volatile delivery
would be an important astrobiological mechanism and may be observable as
atmospheric trace gases.Comment: 11 pages, 8 figures, accepted for publication in
Astronomy&Astrophysic
Molecular Inventories and Chemical Evolution of Low-mass Protostellar Envelopes
This paper presents the first substantial study of the chemistry of the
envelopes around a sample of 18 low-mass pre- and protostellar objects for
which physical properties have previously been derived from radiative transfer
modeling of their dust continuum emission. Single-dish line observations of 24
transitions of 9 molecular species (not counting isotopes) including HCO+,
N2H+, CS, SO, SO2, HCN, HNC, HC3N and CN are reported. The line intensities are
used to constrain the molecular abundances by comparison to Monte Carlo
radiative transfer modeling of the line strengths. An empirical chemical
network is constructed on the basis of correlations between the abundances of
various species. For example, it is seen that the HCO+ and CO abundances are
linearly correlated, both increasing with decreasing envelope mass. Species
such as CS, SO and HCN show no trend with envelope mass. In particular no trend
is seen between ``evolutionary stage'' of the objects and the abundances of the
main sulfur- or nitrogen-containing species. Among the nitrogen-bearing species
abundances of CN, HNC and HC3N are found to be closely correlated, which can be
understood from considerations of the chemical network. The CS/SO abundance
ratio is found to correlate with the abundances of CN and HC3N, which may
reflect a dependence on the atomic carbon abundance. An anti-correlation is
found between the deuteration of HCO+ and HCN, reflecting different temperature
dependences for gas-phase deuteration mechanisms. The abundances are compared
to other protostellar environments. In particular it is found that the
abundances in the cold outer envelope of the previously studied class 0
protostar IRAS16293-2422 are in good agreement with the average abundances for
the presented sample of class 0 objects.Comment: Accepted for publication in A&A. 29 pages, 23 figures. Abstract
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