210 research outputs found
ISO-SWS spectroscopy of gas-phase C2H2 and HCN toward massive YSOs
Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe
The characteristics of the IR emission features in the spectra of Herbig Ae stars: Evidence for chemical evolution
Herbig Ae/Be stars are a class of young pre-main sequence stellar objects of
intermediate mass and are known to have varying amounts of natal cloud material
still present in their direct vicinity. We characterise the IR emission bands,
due to fluorescence by PAH molecules, in the spectra of Herbig Ae/Be stars and
link observed variations to spatial aspects of the mid-IR emission. We analysed
two PAH dominated spectra from a sample of 15 Herbig Ae/Be stars observed with
Spitzer and derive profiles of the major PAH bands. The shape and the measured
band characteristics show pronounced variations between the two Spitzer
spectra. Those variations parallel those found between three ISO spectra of
other, well-studied, Herbig Ae/Be stars. The derived profiles are compared to
those from a broad sample of sources. The Spitzer and ISO spectra exhibit
characteristics commonly interpreted respectively as interstellar matter-like
(ISM), non-ISM-like, or a combination of the two. We argue that the PAH
emission detected from the sources exhibiting a combination of ISM-like and
non-ISM-like characteristics indicates the presence of two dissimilar,
spatially separated, PAH families. As the shape of the individual PAH band
profiles reflects the composition of the PAH molecules involved, this
demonstrates that PAHs in subsequent, evolutionary linked stages of star
formation are different from those in the general ISM, implying active
chemistry. None of the detected PAH emission can be associated with the
(unresolved) disk and is thus associated with the circumstellar cloud. This
implies that chemical changes may already occur in the natal cloud and not
necessarily in the disk
c2d Spitzer IRS spectra of embedded low-mass young stars: gas-phase emission lines
A survey of mid-IR gas-phase emission lines of H2, H2O and various atoms
toward a sample of 43 embedded low-mass young stars in nearby star-forming
regions is presented. The sources are selected from the Spitzer "Cores to
Disks" (c2d) legacy program. The environment of embedded protostars is complex
both in its physical structure (envelopes, outflows, jets, protostellar disks)
and the physical processes (accretion, irradiation by UV and/or X-rays,
excitation through slow and fast shocks) which take place. A key point is to
spatially resolve the emission in the Spitzer-IRS spectra. An optimal
extraction method is used to separate both spatially unresolved (compact, up to
a few 100 AU) and spatially resolved (extended, 1000 AU or more) emission from
the IRS spectra. The results are compared with the c2d disk sample and
literature PDR and shock models to address the physical nature of the sources.
Both compact and extended emission features are observed. Warm (Tex few 100 K)
H2, observed through the pure rotational H2 S(0), S(1) and S(2) lines, and [S
I] 25 mu emission is observed primarily in the extended component. [S I] is
observed uniquely toward truly embedded sources and not toward disks. On the
other hand hot (Tex>700 K) H2, observed primarily through the S(4) line, and
[Ne II] emission is seen mostly in the compact component. [Fe II] and [Si II]
lines are observed in both spatial components. Hot H2O emission is found in the
compact component of some sources. The observed emission on >=1000 AU scales is
characteristic of PDR emission and likely originates in the outflow cavities in
the remnant envelope created by the stellar wind and jets from the embedded
young stars. Weak shocks along the outflow wall can also contribute. The
compact emission is likely of mixed origin, comprised of optically thick
circumstellar disk and/or jet/outflow emission from the protostellar object.Comment: 22 pages, 11 figures, accepted for publication in A&
Hot Organic Molecules Toward a Young Low-Mass Star: A Look at Inner Disk Chemistry
Spitzer Space Telescope spectra of the low mass young stellar object (YSO)
IRS 46 (L_bol ~ 0.6 L_sun) in Ophiuchus reveal strong vibration-rotation
absorption bands of gaseous C2H2, HCN, and CO2. This is the only source out of
a sample of ~100 YSO's that shows these features and the first time they are
seen in the spectrum of a solar-mass YSO. Analysis of the Spitzer data combined
with Keck L- and M-band spectra gives excitation temperatures of > 350 K and
abundances of 10(-6)-10(-5) with respect to H2, orders of magnitude higher than
those found in cold clouds. In spite of this high abundance, the HCN J=4-3 line
is barely detected with the James Clerk Maxwell Telescope, indicating a source
diameter less than 13 AU. The (sub)millimeter continuum emission and the
absence of scattered light in near-infrared images limits the mass and
temperature of any remnant collapse envelope to less than 0.01 M_sun and 100 K,
respectively. This excludes a hot-core type region as found in high-mass YSO's.
The most plausible origin of this hot gas rich in organic molecules is in the
inner (<6 AU radius) region of the disk around IRS 46, either the disk itself
or a disk wind. A nearly edge-on 2-D disk model fits the spectral energy
distribution (SED) and gives a column of dense warm gas along the line of sight
that is consistent with the absorption data. These data illustrate the unique
potential of high-resolution infrared spectroscopy to probe organic chemistry,
gas temperatures and kinematics in the planet-forming zones close to a young
star.Comment: 4 pages, 4 figures; To appear in Astrophysical Journal Letter
Detection of Acetylene toward Cepheus A East with Spitzer
The first map of interstellar acetylene (C2H2) has been obtained with the
infrared spectrograph onboard the Spitzer Space Telescope. A spectral line map
of the vibration-rotation band at 13.7 microns carried out toward the
star-forming region Cepheus A East, shows that the C2H2 emission peaks in a few
localized clumps where gas-phase CO2 emission was previously detected with
Spitzer. The distribution of excitation temperatures derived from fits to the
C2H2 line profiles ranges from 50 to 200 K, a range consistent with that
derived for gaseous CO2 suggesting that both molecules probe the same warm gas
component. The C2H2 molecules are excited via radiative pumping by 13.7 microns
continuum photons emanating from the HW2 protostellar region. We derive column
densities ranging from a few x 10^13 to ~ 7 x 10^14 cm^-2, corresponding to
C2H2 abundances of 1 x 10^-9 to 4 x 10^-8 with respect to H2. The spatial
distribution of the C2H2 emission along with a roughly constant N(C2H2)/N(CO2)
strongly suggest an association with shock activity, most likely the result of
the sputtering of acetylene in icy grain mantles.Comment: 11 pages, 5 figures, accepted for publication in ApJ Letter
H2O and OH gas in the terrestrial planet-forming zones of protoplanetary disks
We present detections of numerous 10-20 micron H2O emission lines from two
protoplanetary disks around the T Tauri stars AS 205A and DR Tau, obtained
using the InfraRed Spectrograph on the Spitzer Space Telescope. Follow-up 3-5
micron Keck-NIRSPEC data confirm the presence of abundant water and spectrally
resolve the lines. We also detect the P4.5 (2.934 micron) and P9.5 (3.179
micron) doublets of OH and 12CO/13CO v=1-0 emission in both sources. Line
shapes and LTE models suggest that the emission from all three molecules
originates between ~0.5 and 5 AU, and so will provide a new window for
understanding the chemical environment during terrestrial planet formation. LTE
models also imply significant columns of H2O and OH in the inner disk
atmospheres, suggesting physical transport of volatile ices either vertically
or radially; while the significant radial extent of the emission stresses the
importance of a more complete understanding of non-thermal excitation
processes.Comment: 9 pages, 3 figures, 1 table, aastex, to appear in the Astrophysical
Journa
The c2d Spitzer spectroscopy survey of ices around low-mass young stellar objects, III: CH4
CH4 is proposed to be the starting point of a rich organic chemistry. Solid
CH4 abundances have previously been determined mostly toward high mass star
forming regions. Spitzer/IRS now provides a unique opportunity to probe solid
CH4 toward low mass star forming regions as well. Infrared spectra from the
Spitzer Space Telescope are presented to determine the solid CH4 abundance
toward a large sample of low mass young stellar objects. 25 out of 52 ice
sources in the (cores to disks) legacy have an absorption feature at 7.7
um, attributed to the bending mode of solid CH4. The solid CH4 / H2O abundances
are 2-8%, except for three sources with abundances as high as 11-13%. These
latter sources have relatively large uncertainties due to small total ice
column densities. Toward sources with H2O column densities above 2E18 cm-2, the
CH4 abundances (20 out of 25) are nearly constant at 4.7+/-1.6%. Correlation
plots with solid H2O, CH3OH, CO2 and CO column densities and abundances
relative to H2O reveal a closer relationship of solid CH4 with CO2 and H2O than
with solid CO and CH3OH. The inferred solid CH4 abundances are consistent with
models where CH4 is formed through sequential hydrogenation of C on grain
surfaces. Finally the equal or higher abundances toward low mass young stellar
objects compared with high mass objects and the correlation studies support
this formation pathway as well, but not the two competing theories: formation
from CH3OH and formation in gas phase with subsequent freeze-out.Comment: 27 pages, 7 figures, accepted by Ap
Gas-phase SO2 in absorption towards massive protostars
We present the first detection of the v(3) ro-vibrational band of gas-phase SO2 in absorption in the mid-infrared spectral region around 7.3 mum of a sample of deeply embedded massive protostars. Comparison with model spectra shows that the derived excitation temperatures correlate with previous C2H2 and HCN studies, indicating that the same warm gas component is probed. The SO2 column densities are similar along all lines of sight suggesting that the SO2 formation has saturated, but not destroyed, and the absolute abundances of SO2 are high (similar to 10(7)). Both the high temperature and the high abundance of the detected SO2 are not easily explained by standard hot core chemistry models. Likewise, indicators of shock induced chemistry are lacking
Modeling Spitzer observations of VV Ser. I. The circumstellar disk of a UX Orionis star
We present mid-infrared Spitzer-IRS spectra of the well-known UX Orionis star
VV Ser. We combine the Spitzer data with interferometric and spectroscopic data
from the literature covering UV to submillimeter wavelengths. The full set of
data are modeled by a two-dimensional axisymmetric Monte Carlo radiative
transfer code. The model is used to test the prediction of (Dullemond et al.
2003) that disks around UX Orionis stars must have a self-shadowed shape, and
that these disks are seen nearly edge-on, looking just over the edge of a
puffed-up inner rim, formed roughly at the dust sublimation radius. We find
that a single, relatively simple model is consistent with all the available
observational constraints spanning 4 orders of magnitude in wavelength and
spatial scales, providing strong support for this interpretation of UX Orionis
stars. The grains in the upper layers of the puffed-up inner rim must be small
(0.01-0.4 micron) to reproduce the colors (R_V ~ 3.6) of the extinction events,
while the shape and strength of the mid-infrared silicate emission features
indicate that grains in the outer disk (> 1-2 AU) are somewhat larger (0.3-3.0
micron). From the model fit, the location of the puffed-up inner rim is
estimated to be at a dust temperature of 1500 K or at 0.7-0.8 AU for small
grains. This is almost twice the rim radius estimated from near-infrared
interferometry. A best fitting model for the inner rim in which large grains in
the disk mid-plane reach to within 0.25 AU of the star, while small grains in
the disk surface create a puffed-up inner rim at ~0.7-0.8 AU, is able to
reproduce all the data, including the near-infrared visibilities. [Abstract
abridged]Comment: 12 pages, accepted for publication in Ap
- …