891 research outputs found
Detection of steam in the circumstellar disk around a massive Young Stellar Object
We report on the observation of hot water vapor (steam) in the inner AU of a
young massive star located in the star-forming region IRAS 08576-4334. The
water lines are detected in a medium resolution (R=10,000) K-band spectrum
taken by the infrared spectrometer ISAAC mounted on the VLT-ANTU. The water
vapor is at a mean temperature of 1565+/-510 K, cooler than the hot CO found in
the same object, which is at 1660K and the column density is N(H2O)=(2.5 +/-
0.4)x1E18 cm-2. The profile of both H2O and CO lines is best reproduced by the
emission from a Keplerian disk. To interpret the data, we also investigate the
formation of molecules and especially CO and water vapor in the inner hot and
dense part of disks around young high mass stars using a pseudo time-dependent
gas-phase chemical model. Molecules are rapidly photodissociated but this
destruction is compensated by an efficient formation due to fast
neutral-neutral reactions. The ability of CO molecules to self-shield
significantly enhances its abundance. Water molecules are sufficiently abundant
to be detectable. The observed H2O/CO ratio is reproduced by gas at 1600K and
an enhanced UV field over gas density ratio I_UV/nH=1E(-4)- 1E(-6). The
simulations support the presence of CO and H2O molecules in the inner disks
around young massive stars despite the strong UV radiation and show that the OH
radical plays an essential role in hot gas chemistry.Comment: 15 pages, 11 figures, Accepted to A&
VLT/MUSE view of the highly ionized outflow cones in the nearby starburst ESO338-IG04
The Ly line is an important diagnostic for star formation at high
redshift, but interpreting its flux and line profile is difficult because of
the resonance nature of Ly. Trends between the escape of Ly
photons and dust and properties of the interstellar medium (ISM) have been
found, but detailed comparisons between Ly emission and the properties
of the gas in local high-redshift analogs are vital for understanding the
relation between Ly emission and galaxy properties. For the first time,
we can directly infer the properties of the ionized gas at the same location
and similar spatial scales of the extended Ly halo around ESO 338-IG04.
We obtained VLT/MUSE integral field spectra. We used ionization parameter
mapping of the [SII]/[OIII] line ratio and the kinematics of H to study
the ionization state and kinematics of the ISM of ESO338-IG04. The velocity map
reveals two outflows. The entire central area of the galaxy is highly ionized
by photons leaking from the HII regions around the youngest star clusters.
Three highly ionized cones have been identified, of which one is associated
with an outflow. We propose a scenario where the outflows are created by
mechanical feedback of the older clusters, while the highly ionized gas is
caused by the hard ionizing photons emitted by the youngest clusters. A
comparison with the Ly map shows that the (approximately bipolar)
asymmetries observed in the Ly emission are consistent with the base of
the outflows detected in H. No clear correlation with the ionization
cones is found. The mechanical and ionization feedback of star clusters
significantly changes the state of the ISM by creating ionized cones and
outflows. The comparison with Ly suggests that especially the outflows
could facilitate the escape of Ly photons [Abridged].Comment: Accepted for publication in A&A Letters, 4 pages, 2 figure
A dearth of short-period massive binaries in the young massive star forming region M17: Evidence for a large orbital separation at birth?
The formation of massive stars remains poorly understood and little is known
about their birth multiplicity properties. Here, we investigate the strikingly
low radial-velocity dispersion measured for a sample of 11 massive pre- and
near-main-sequence stars (sigma_rv = 5.6 +/- 0.2 km/s) in the young massive
star forming region M17 to obtain first constraints on the multiplicity
properties of young massive stellar objects. Methods: We compute the RV
dispersion of synthetic populations of massive stars for various multiplicity
properties and we compare the simulated sigma_rv distributions to the observed
value. We specifically investigate two scenarios: a low binary fraction and a
dearth of short-period binary systems. Results: Simulated populations with low
binary fractions (f_bin = 0.12_{-0.09}^{+0.16}) or with truncated period
distributions (P_cutoff > 9 months) are able to reproduce the low sigma_rv
observed within their 68%-confidence intervals. Parent populations with f_bin >
0.42 or P_cutoff < 47 d can however be rejected at the 5%-significance level.
Both constraints are contrast with the high binary fraction and plethora of
short-period systems found in few Myr-old, OB-type populations. To explain the
difference, the first scenario requires a variation of the outcome of the
massive star formation process. In the the second scenario, compact binaries
must form later on, and the cut-off period may be related to physical
length-scales representative of the bloated pre-main-sequence stellar radii or
of their accretion disks. Conclusions: If the obtained constraints are
representative of the overall properties of massive young stellar objects, our
results may provide support to a formation process in which binaries are
initially formed at larger separations, then harden or migrate to produce the
typical (untruncated) power-law period distribution observed in few Myr-old OB
binaries.Comment: 5 pages; Accepted for publication in Astronomy and Astrophysics
Letter
Super star cluster feedback driving ionization, shocks and outflows in the halo of the nearby starburst ESO 338-IG04
Stellar feedback strongly affects the interstellar medium (ISM) of galaxies.
Stellar feedback in the first galaxies likely plays a major role in enabling
the escape of LyC photons, which contribute to the re-ionization of the
Universe. Nearby starburst galaxies serve as local analogues allowing for a
spatially resolved assessment of the feedback processes in these galaxies. We
characterize the feedback effects from the star clusters in the local
high-redshift analogue ESO 338-IG04 on the ISM and compare the results with the
properties of the most massive clusters. We use high quality VLT/MUSE optical
integral field data to derive the physical properties of the ISM such as
ionization, density, shocks, and perform new fitting of the spectral energy
distributions of the brightest clusters in ESO 338-IG04 from HST imaging. ESO
338-IG04 has a large ionized halo which we detect to a distance of 9 kpc. We
identify 4 Wolf-Rayet (WR) clusters based on the blue and red WR bump. We
follow previously identified ionization cones and find that the ionization of
the halo increases with distance. Analysis of the galaxy kinematics shows two
complex outflows driven by the numerous young clusters in the galaxy. We find a
ring of shocked emission traced by an enhanced [OI]/H ratio surrounding
the starburst and at the end of the outflow. Finally we detect nitrogen
enriched gas associated with the outflow, likely caused by the WR stars in the
massive star clusters. Photo-ionization dominates the central starburst and
sets the ionization structure of the entire halo, resulting in a density
bounded halo, facilitating the escape of LyC photons. Outside the central
starburst, shocks triggered by an expanding super bubble become important. The
shocks at the end of the outflow suggest interaction between the hot outflowing
material and the more quiescent halo gas.Comment: Accepted for publication in Astronomy and Astrophysics, 22 pages, 15
figure
Evidence for an inner molecular disk around massive Young Stellar Objects
We present observations of CO overtone bandhead emission toward four massive
Young Stellar Objects (spectral type O6--B5). The high signal-to-noise ratio
K-band spectra were obtained with VLT-ISAAC at a resolution of 30 km/s,
sufficient to resolve the bandheads, but not the individual J-lines. We are
able to explain the shape of the lines by assuming a simple isothermal
keplerian disk model seen at different inclinations. The gas temperature ranges
from 1500 to 4500 K and the CO column density is between 0.1 and 4 10^21 cm^-2.
The emission probably arises within the first few astronomical units of the
disk, consistent with the high gas temperature. Our results indicate that
molecules can survive close to a hot star and suggest that dense (n_H > 10^10
cm^-3) inner disks may be relatively common at an advanced stage of high-mass
star formation.Comment: accepted for publication in Astronomy and Astrophysics Letter
MUSE Illuminates Channels for Lyman Continuum Escape in the Halo of SBS 0335-52E
We report on the discovery of ionised gas filaments in the circum-galactic
halo of the extremely metal-poor compact starburst SBS 0335-052E in a 1.5h
integration with the MUSE integral-field spectrograph. We detect these features
in H and [OIII] emission down to surface-brightness levels of erg scmarcsec. The filaments have
projected diameters of 2.1 kpc and extend more than 9 kpc to the north and
north-west from the main stellar body. We also detect extended nebular HeII
4686 emission that brightens towards the north-west at the rim of a
star-burst driven super-shell, suggestive of a locally enhanced UV radiation
field due to shocks. We also present a velocity field of the ionised gas. The
filaments appear to connect seamlessly in velocity space to the kinematical
disturbances caused by the shell. Similar to high- star-forming galaxies,
the ionised gas in this galaxy is dispersion dominated. We argue that the
filaments were created via feedback from the starburst and that these ionised
structures in the halo may act as escape channels for Lyman continuum radiation
in this gas-rich system.Comment: Revised version after peer review. Accepted for publication in A&A
letter
Near-infrared spectroscopy in NGC 7538
The characterisation of the stellar population toward young high-mass
star-forming regions allows to constrain fundamental cluster properties like
distance and age. These are essential when using high-mass clusters as probes
to conduct Galactic studies. NGC 7538 is a star-forming region with an embedded
stellar population only unearthed in the near-infrared. We present the first
near-infrared spectro-photometric study of the candidate high-mass stellar
content in NGC 7538. We obtained H and K spectra of 21 sources with both the
multi-object and long-slit modes of LIRIS at the WHT, and complement these data
with sub-arcsecond JHKs photometry of the region using the imaging mode of the
same instrument. We find a wide variety of objects within the studied stellar
population of NGC 7538. Our results discriminate between a stellar population
associated to the HII region, but not contained within its extent, and several
pockets of more recent star formation. We report the detection of CO bandhead
emission toward several sources as well as other features indicative of a young
stellar nature. We infer a spectro-photometric distance of 2.7+-0.5 kpc, an age
spread in the range 0.5-2.2 Myr and a total mass ~1.7x10^3 Msun for the older
population.Comment: 11 pages, 8 figures, 1 table, accepted by A&
Continuous star cluster formation in the spiral NGC 45
We determined ages for 52 star clusters with masses < 10^6 solar masses in
the low surface brightness spiral galaxy NGC 45. Four of these candidates are
old globular clusters located in the bulge. The remaining ones span a large age
range. The cluster ages suggest a continuous star/cluster formation history
without evidence for bursts, consistent with the galaxy being located in a
relatively unperturbed environment in the outskirts of the Sculptor group.Comment: 4 pages, 3 figures. To appear in "Island Universes - Structure and
Evolution of Disk Galaxies", Terschelling (Netherlands), July 200
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