492 research outputs found
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
Results from a VLT-ISAAC survey of ices and gas around young stellar objects
General results from a 3-5 micron spectroscopic survey of nearby low-mass
young stellar objects are presented. L and M-band spectra have been obtained of
\~50 low mass embedded young stars using the ISAAC spectrometer mounted on
UT1-Antu at Paranal Observatory. For the first time, a consistent census of the
CO, H2O ices and the minor ice species CH3OH and OCN- and warm CO gas present
around young stars is obtained, using large number statistics and resolving
powers of up to R=10000. The molecular structure of circumstellar CO ices, the
depletion of gaseous CO onto grains in protoplanetary disks, the presence of
hot gas in the inner parts of circumstellar disks and in outflows and infalls
are studied. Furthermore, the importance of scattering effects for the
interpretation of the spectra have been addressed.Comment: To appear in the proceedings of the conference "Chemistry as a
Diagnostic of Star Formation", University of Waterloo, Canada, 21-23 August
200
VLT-ISAAC 3-5 micron spectroscopy of low-mass young stellar objects: prospects for CRIRES
We present results from an extensive spectroscopic survey in the 3-5 micron
wavelength region of low-mass young stellar objects using VLT-ISAAC. Medium
resolution spectra (R ~ 1000-10000) of young embedded stars in the mid-infrared
allow for detailed studies of ro-vibrational lines from molecular gas,
interstellar ices and Polycyclic Aromatic Hydrocarbons (PAHs). By taking
advantage of this wide range of molecular tracers available within a few
spectral settings, the survey has helped to constrain the chemical evolution of
cold molecular material in low-mass star forming regions as well as the physics
of disks surrounding protostars. In this contribution, we will review the
various spectral diagnostics of molecular material, which require ground-based
high resolution infrared spectroscopy. The importance of a high resolution
spectroscopic capability as will be offered by CRIRES is discussed in the
context of the physics and chemistry of low-mass star formation.Comment: 11 pages, Proceedings of the ESO workshop: "High Resolution Infrared
Spectroscopy in Astronomy", H.U. Kaufl, R. Siebenmorgen & A. Moorwood (eds.),
Garching, Germany, November 200
Lack of PAH emission toward low-mass embedded young stellar objects
PAHs have been detected toward molecular clouds and some young stars with
disks, but have not yet been associated with embedded young stars. We present a
sensitive mid-IR spectroscopic survey of PAH features toward a sample of
low-mass embedded YSOs. The aim is to put constraints on the PAH abundance in
the embedded phase of star formation using radiative transfer modeling.
VLT-ISAAC L-band spectra for 39 sources and Spitzer IRS spectra for 53
sources are presented. Line intensities are compared to recent surveys of
Herbig Ae/Be and T Tauri stars. The radiative transfer codes RADMC and RADICAL
are used to model the PAH emission from embedded YSOs consisting of a PMS star
with a circumstellar disk embedded in an envelope. The dependence of the PAH
feature on PAH abundance, stellar radiation field, inclination and the
extinction by the surrounding envelope is studied.
The 3.3 micron PAH feature is undetected for the majority of the sample
(97%), with typical upper limits of 5E-16 W/m^2. Compact 11.2 micron PAH
emission is seen directly towards 1 out of the 53 Spitzer Short-High spectra,
for a source that is borderline embedded. For all 12 sources with both VLT and
Spitzer spectra, no PAH features are detected in either. In total, PAH features
are detected toward at most 1 out of 63 (candidate) embedded protostars (<~
2%), even lower than observed for class II T Tauri stars with disks (11-14%).
Assuming typical class I stellar and envelope parameters, the absence of PAHs
emission is most likely explained by the absence of emitting carriers through a
PAH abundance at least an order of magnitude lower than in molecular clouds but
similar to that found in disks. Thus, most PAHs likely enter the protoplanetary
disks frozen out in icy layers on dust grains and/or in coagulated form.Comment: 13 pages, 9 figures, accepted for publication in A&
Bright CO ro-vibrational emission lines in the class I source GSS 30 IRS1: Probing the inner disk of a young embedded star
We present a 4.5-4.85 micron R=5000 spectrum of the low mass class I young
stellar object GSS 30 IRS1 (L=25L_sun) in the rho Ophiuchus core, observed with
the infrared spectrometer (ISAAC) on the Very Large Telescope (VLT-UT1). Strong
line emission from the ro-vibrational transitions of 12CO and 13CO is detected.
In total more than 40 distinct lines are seen in the covered region. The line
emission is spatially extended and detected up to 2" = 320 AU from the central
source but is spectrally unresolved (Delta v < 30 km/s). This is the first time
strong emission in the fundamental ro-vibrational band from CO has been
observed from an embedded young stellar object. The line fluxes were modeled
using a 1-dimensional full radiative transfer code, which shows that the
emission is fully consistent with a gas in LTE at a single well constrained
temperature (T=515+/-5 K). Furthermore, the ratios between lines from the two
detected isotopic species of CO show that the 12CO lines must be optically
thick. However, this is inconsistent with the observed spatial extent of the
emission, since this implies such low CO column densities that the lines are
optically thin. A likely solution to the discrepancy is that the lines are
emitted by a smaller more dense region and then scattered in the bipolar cavity
present around the central star. This gives a rough estimate of the total
molecular gas mass of 1-100 M_earth and a physical extent of ~20-100 AU. We
propose that the most likely origin of the line emission is post-shocked gas in
a dense dissociative accretion shock from the inner 10-50 AU of a circumstellar
disk. The presence of a shock capable of dissociating molecules in the disk
will have implications for the chemical evolution of disks around young low
mass stars.Comment: 12 pages, 5 figures, accepted by A&
Anomalous CO2 Ice Toward HOPS-68: A Tracer of Protostellar Feedback
We report the detection of a unique CO2 ice band toward the deeply embedded,
low-mass protostar HOPS-68. Our spectrum, obtained with the Infrared
Spectrograph onboard the Spitzer Space Telescope, reveals a 15.2 micron CO2 ice
bending mode profile that cannot modeled with the same ice structure typically
found toward other protostars. We develop a modified CO2 ice profile
decomposition, including the addition of new high-quality laboratory spectra of
pure, crystalline CO2 ice. Using this model, we find that 87-92% of the CO2 is
sequestered as spherical, CO2-rich mantles, while typical interstellar ices
show evidence of irregularly-shaped, hydrogen-rich mantles. We propose that (1)
the nearly complete absence of unprocessed ices along the line-of-sight is due
to the flattened envelope structure of HOPS-68, which lacks cold absorbing
material in its outer envelope, and possesses an extreme concentration of
material within its inner (10 AU) envelope region and (2) an energetic event
led to the evaporation of inner envelope ices, followed by cooling and
re-condensation, explaining the sequestration of spherical, CO2 ice mantles in
a hydrogen-poor mixture. The mechanism responsible for the sublimation could be
either a transient accretion event or shocks in the interaction region between
the protostellar outflow and envelope. The proposed scenario is consistent with
the rarity of the observed CO2 ice profile, the formation of nearly pure CO2
ice, and the production of spherical ice mantles. HOPS-68 may therefore provide
a unique window into the protostellar feedback process, as outflows and heating
shape the physical and chemical structure of protostellar envelopes and
molecular clouds.Comment: Accepted to the Astrophysical Journal, 2013 February 15: 14 pages, 9
figures, 3 table
Infrared spectroscopy of solid CO-CO2 mixtures and layers
The spectra of pure, mixed and layered CO and CO2 ices have been studied
systematically under laboratory conditions using infrared spectroscopy. This
work provides improved resolution spectra (0.5 cm-1) of the CO2 bending and
asymmetric stretching mode, as well as the CO stretching mode, extending the
existing Leiden database of laboratory spectra to match the spectral resolution
reached by modern telescopes and to support the interpretation of the most
recent data from Spitzer. It is shown that mixed and layered CO and CO2 ices
exhibit very different spectral characteristics, which depend critically on
thermal annealing and can be used to distinguish between mixed, layered and
thermally annealed CO-CO2 ices. CO only affects the CO2 bending mode spectra in
mixed ices below 50K under the current experimental conditions, where it
exhibits a single asymmetric band profile in intimate mixtures. In all other
ice morphologies the CO2 bending mode shows a double peaked profile, similar to
that observed for pure solid CO2. Conversely, CO2 induces a blue-shift in the
peak-position of the CO stretching vibration, to a maximum of 2142 cm-1 in
mixed ices, and 2140-2146 cm-1 in layered ices. As such, the CO2 bending mode
puts clear constraints on the ice morphology below 50K, whereas beyond this
temperature the CO2 stretching vibration can distinguish between initially
mixed and layered ices. This is illustrated for the low-mass YSO HH46, where
the laboratory spectra are used to analyse the observed CO and CO2 band
profiles and try to constrain the formation scenarios of CO2.Comment: Accepted in A&
Quantification of segregation dynamics in ice mixtures
(Abridged) The observed presence of pure CO2 ice in protostellar envelopes is
attributed to thermally induced ice segregation, but a lack of quantitative
experimental data has prevented its use as a temperature probe. Quantitative
segregation studies are also needed to characterize diffusion in ices, which
underpins all ice dynamics and ice chemistry. This study aims to quantify the
segregation mechanism and barriers in different H2O:CO2 and H2O:CO ice mixtures
covering a range of astrophysically relevant ice thicknesses and mixture
ratios. The ices are deposited at 16-50 K under (ultra-)high vacuum conditions.
Segregation is then monitored at 23-70 K as a function of time, through
infrared spectroscopy. Thin (8-37 ML) H2O:CO2/CO ice mixtures segregate
sequentially through surface processes, followed by an order of magnitude
slower bulk diffusion. Thicker ices (>100 ML) segregate through a fast bulk
process. The thick ices must therefore be either more porous or segregate
through a different mechanism, e.g. a phase transition. The segregation
dynamics of thin ices are reproduced qualitatively in Monte Carlo simulations
of surface hopping and pair swapping. The experimentally determined
surface-segregation rates for all mixture ratios follow the Ahrrenius law with
a barrier of 1080[190] K for H2O:CO2 and 300[100] K for H2O:CO mixtures. During
low-mass star formation H2O:CO2 segregation will be important already at 30[5]
K. Both surface and bulk segregation is proposed to be a general feature of ice
mixtures when the average bond strengths of the mixture constituents in pure
ice exceeds the average bond strength in the ice mixture.Comment: Accepted for publication in A&A. 25 pages, including 13 figure
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