47 research outputs found
Spitzer spectral line mapping of protostellar outflows: II H2 emission in L1157
We present an analysis of Spitzer-IRS spectroscopic maps of the L1157
protostellar outflow in the H2 pure-rotational lines from S(0) to S(7). The aim
of this work is to derive the physical conditions pertaining to the warm
molecular gas and study their variations within the flow. The mid-IR H2
emission follows the morphology of the precessing flow, with peaks correlated
with individual CO clumps and H2 2.12{\mu}m ro-vibrational emission. More
diffuse emission delineating the CO cavities is detected only in the low-laying
transitions, with J(lower) less or equal to 2. The H2 line images have been
used to construct 2D maps of N(H2), H2 ortho-to-para ratio and temperature
spectral index beta, in the assumption of a gas temperature stratification
where the H2 column density varies as T^(beta). Variations of these parameters
are observed along the flow. In particular, the ortho-to-para ratio ranges from
0.6 to 2.8, highlighting the presence of regions subject to recent shocks where
the ortho-to-para ratio has not had time yet to reach the equilibrium value.
Near-IR spectroscopic data on ro-vibrational H2 emission have been combined
with the mid-IR data and used to derive additional shock parameters in the
brightest blue- and red-shifted emission knots. A high abundance of atomic
hydrogen (H/H2 about 0.1-0.3) is implied by the observed H2 column densities,
assuming n(H2) values as derived by independent SiO observations. The presence
of a high fraction of atomic hydrogen, indicates that a partially-dissociative
shock component should be considered for the H2 excitation in these localized
regions. However, planar shock models, either of C- or J-type, are not able to
consistently reproduce all the physical parameters derived from our analysis of
the H2 emission. Globally, H2 emission contributes to about 50% of the total
shock radiated energy in the L1157 outflow.Comment: 31 pages, 9 figure, Accepted for publication on Ap
Hydrogen permitted lines in the first near-IR spectra of Th 28 microjet: accretion or ejection tracers?
We report the first near-infrared detection of the bipolar microjet from
TTauri star ThA 15-28 (aka Th 28). Spectra were obtained with VLT/ISAAC for the
slit both perpendicular and parallel to the flow to examine jet kinematics and
gas physics within the first arcsecond from the star. The jet was successfully
detected in both molecular and atomic lines. The H_2 component was found to be
entirely blueshifted around the base of the bipolar jet. It shows that only the
blue lobe is emitting in H_2 while light is scattered in the direction of the
red lobe, highlighting an asymmetric extinction and/or excitation between the
two lobes. Consistent with this view, the red lobe is brighter in all atomic
lines. Interestingly, the jet was detected not only in [Fe II], but also in Br
gamma and Pa beta lines. Though considered tracers mainly of accretion, we find
that these high excitation hydrogen permitted lines trace the jet as far as 150
AU from the star. This is confirmed in a number of ways: the presence of the
[Fe II] 2.13 micron line which is of similarly high excitation; H I velocities
which match the jet [Fe II] velocities in both the blue and red lobe; and high
electron density close to the source of >6x10^4 cm^-3 derived from the [Fe II]
1.64,1.60 micron ratio. These near-infrared data complement HST/STIS optical
and near-ultraviolet data for the same target which were used in a jet rotation
study, although no rotation signature could be identified here due to
insufficient angular resolution. The unpublished HST/STIS H alpha emission is
included here along side the other H I lines. Identifying Br gamma and Pa beta
as tracers of ejection is significant because of the importance of finding
strong near-infrared probes close to the star, where forbidden lines are
quenched, which will help understand accretion-ejection when observed with high
spatial resolution instruments such as VLTI/AMBER.Comment: 18 pages, 26 figures, Accepted by Ap
The water abundance behind interstellar shocks: results from /PACS and /IRS observations of HO, CO, and H
We have investigated the water abundance in shock-heated molecular gas,
making use of measurements of far-infrared CO and HO line
emissions in combination with measurements of mid-IR H rotational
emissions. We present far-infrared line spectra obtained with 's PACS
instrument in range spectroscopy mode towards two positions in the protostellar
outflow NGC 2071 and one position each in the supernova remnants W28 and 3C391.
These spectra provide unequivocal detections, at one or more positions, of 12
rotational lines of water, 14 rotational lines of CO, 8 rotational lines of OH
(4 lambda doublets), and 7 fine-structure transitions of atoms or atomic ions.
We first used a simultaneous fit to the CO line fluxes, along with H
rotational line fluxes measured previously by , to constrain the
temperature and density distribution within the emitting gas; and we then
investigated the water abundances implied by the observed HO line fluxes.
The water line fluxes are in acceptable agreement with standard theoretical
models for nondissociative shocks that predict the complete vaporization of
grain mantles in shocks of velocity km/s, behind which the
characteristic gas temperature is K and the HO/CO ratio is 1.2Comment: 42 pages, 15 figures, accepted for publication in the Astrophysical
Journa
Accretion and outflows in young stars with CUBES
The science case on studies of accretion and outflows in low-mass (<1.5M⊙) young stellar objects (YSOs) with the new CUBES instrument is presented. We show the need for a high-sensitivity, near-ultraviolet (NUV) spectrograph like CUBES, with a resolving power at least four times that of X-Shooter and combined with UVES via a fibrelink for simultaneous observations. Simulations with the CUBES exposure time calculator and the end-to-end software show that a significant gain in signal-to-noise can be achieved compared to current instruments, for both the spectral continuum and emission lines, including for relatively embedded YSOs. Our simulations also show that the low-resolution mode of CUBES will be able to observe much fainter YSOs (V ∼ 22 mag) in the NUV than we can today, allowing us extend studies to YSOs with background-limited magnitudes. The performance of CUBES in terms of sensitivity in the NUV will provide important new insights into the evolution of circumstellar disks, by studying the accretion, jets/winds and photo-evaporation processes, down to the low-mass brown dwarf regime. CUBES will also open-up new science as it will be able to observe targets that are several magnitudes fainter than those reachable with current instruments, facilitating studies of YSOs at distances of ∼ kpc scale. This means a step-change in the field of low-mass star formation, as it will be possible to expand the science case from relatively local star-forming regions to a large swathe of distances within the Milky Way
Spitzer spectral line mapping of protostellar outflows: I. Basic data and outflow energetics
We report the results of spectroscopic mapping observations carried out
toward protostellar outflows in the BHR71, L1157, L1448, NGC 2071, and VLA 1623
molecular regions using the Infrared Spectrograph (IRS) of the Spitzer Space
Telescope. These observations, covering the 5.2 - 37 micron spectral region,
provide detailed maps of the 8 lowest pure rotational lines of molecular
hydrogen and of the [SI] 25.25 micron and [FeII] 26.0 micron fine structure
lines. The molecular hydrogen lines, believed to account for a large fraction
of the radiative cooling from warm molecular gas that has been heated by a
non-dissociative shock, allow the energetics of the outflows to be elucidated.
Within the regions mapped towards these 5 outflow sources, total H2
luminosities ranging from 0.02 to 0.75 L(solar) were inferred for the sum of
the 8 lowest pure rotational transitions. By contrast, the much weaker [FeII]
26.0 micron fine structure transition traces faster, dissociative shocks; here,
only a small fraction of the fast shock luminosity emerges as line radiation
that can be detected with Spitzer/IRS.Comment: 38 pages including 17 figures. Accepted for publication in Ap
First detection of water vapor in a pre-stellar core
Water is a crucial molecule in molecular astrophysics as it controls much of
the gas/grain chemistry, including the formation and evolution of more complex
organic molecules in ices. Pre-stellar cores provide the original reservoir of
material from which future planetary systems are built, but few observational
constraints exist on the formation of water and its partitioning between gas
and ice in the densest cores. Thanks to the high sensitivity of the Herschel
Space Observatory, we report on the first detection of water vapor at high
spectral resolution toward a dense cloud on the verge of star formation, the
pre-stellar core L1544. The line shows an inverse P-Cygni profile,
characteristic of gravitational contraction. To reproduce the observations,
water vapor has to be present in the cold and dense central few thousand AU of
L1544, where species heavier than Helium are expected to freeze-out onto dust
grains, and the ortho:para H2 ratio has to be around 1:1 or larger. The
observed amount of water vapor within the core (about 1.5x10^{-6} Msun) can be
maintained by Far-UV photons locally produced by the impact of galactic cosmic
rays with H2 molecules. Such FUV photons irradiate the icy mantles, liberating
water wapor in the core center. Our Herschel data, combined with radiative
transfer and chemical/dynamical models, shed light on the interplay between gas
and solids in dense interstellar clouds and provide the first measurement of
the water vapor abundance profile across the parent cloud of a future
solar-type star and its potential planetary system.Comment: The Astrophysical Journal Letters, in pres
The Radial Distribution and Excitation of H2 around Young Stars in the HST-ULLYSES Survey
The spatial distribution and evolution of gas in the inner 10 au of
protoplanetary disks form the basis for estimating the initial conditions of
planet formation. Among the most important constraints derived from
spectroscopic observations of the inner disk are the radial distributions of
the major gas phase constituents, how the properties of the gas change with
inner disk dust evolution, and how chemical abundances and excitation
conditions are influenced by the high-energy radiation from the central star.
We present a survey of the radial distribution, excitation, and evolution of
inner disk molecular hydrogen (H) obtained as part of the /ULLYSES
program. We analyze far-ultraviolet spectroscopy of 71 (63 accreting) pre-main
sequence systems in the ULLYSES DR5 release to characterize the H
emission lines, H dissociation continuum emission, and major
photochemical/disk evolution driving UV emissions (Ly, UV continuum,
and C IV). We use the widths of the H emission lines to show that most
fluorescent H arises between 0.1 - 1.4 au from the parent star, and show
positive correlations of the average emitting radius with the accretion
luminosity and with the dust disk mass. We find a strong correlation between
H dissociation emission and both the accretion-dominated Ly
luminosity and the inner disk dust clearing, painting a picture where water
molecules in the inner 3 au are exposed to and dissociated by strong Ly
emission as the opacity of the inner disk declines with time.Comment: 19 pages, 11 figures. Accepted to the Astronomical Journa
High-resolution TNG spectra of T Tauri stars. Near-IR GIANO observations of the young variables XZ Tauri and DR Tauri
Aims: We aim to characterise the star-disk interaction region in T Tauri stars that show photometric and spectroscopic variability. Methods: We used the GIANO instrument at the Telescopio Nazionale Galileo to obtain near-infrared high-resolution spectra (R 50 000) of XZ Tau and DR Tau, which are two actively accreting T Tauri stars classified as EXors. Equivalent widths and profiles of the observed features are used to derive information on the properties of the inner disk, the accretion columns, and the winds. Results: Both sources display composite H I line profiles, where contributions from both accreting gas and high-velocity winds can be recognised. These lines are progressively more symmetric and narrower with increasing upper energy which may be interpreted in terms of two components with different decrements or imputed to self-absorption effects. XZ Tau is observed in a relatively high state of activity with respect to literature observations. The variation of the He I 1.08 μm line blue-shifted absorption, in particular, suggests that the inner wind has undergone a dramatic change in its velocity structure, connected with a recent accretion event. DR Tau has a more stable wind as its He I 1.08 μm absorption does not show variations with time in spite of strong variability of the emission component. The IR veiling in the two sources can be interpreted as due to blackbody emission at temperatures of 1600 K and 2300 K for XZ Tau and DR Tau, respectively, with emitting areas 30 times larger than the central star. While for XZ Tau these conditions are consistent with emission from the inner rim of the dusty disk, the fairly high temperature inferred for DR Tau might suggest that its veiling originates from a thick gaseous disk located within the dust sublimation radius. Strong and broad metallic lines, mainly from C I and Fe I, are detected in XZ Tau, similar to those observed in other EXor sources during burst phases. At variance, DR Tau shows weaker and narrower metallic lines, despite its larger accretion luminosity. This suggests that accretion is not the only driver of metallic line excitation. Conclusions: The presented observations demonstrate the potential of wide-band, high-resolution near-IR spectroscopy to simultaneously probe the different phenomena that occur in the interaction region between the stellar magnetosphere and the accretion disk, thus providing hints on how these two structures are linked to each other