39 research outputs found
Massive pre-main sequence stars in M17
The formation process of massive stars is still poorly understood. Massive
young stellar objects (mYSOs) are deeply embedded in their parental clouds,
they are rare and thus typically distant, and their reddened spectra usually
preclude the determination of their photospheric parameters. M17 is one of the
best studied HII regions in the sky, is relatively nearby, and hosts a young
stellar population. With X-shooter on the ESO Very Large Telescope we have
obtained optical to near-infrared spectra of candidate mYSOs, identified by
Hanson et al. (1997), and a few OB stars in this region. The large wavelength
coverage enables a detailed spectroscopic analysis of their photospheres and
circumstellar disks. We confirm the pre-main sequence (PMS) nature of six of
the stars and characterise the O stars. The PMS stars have radii consistent
with being contracting towards the main sequence and are surrounded by a
remnant accretion disk. The observed infrared excess and the (double-peaked)
emission lines provide the opportunity to measure structured velocity profiles
in the disks. We compare the observed properties of this unique sample of young
massive stars with evolutionary tracks of massive protostars by Hosokawa &
Omukai (2009), and propose that these mYSOs near the western edge of the HII
region are on their way to become main-sequence stars (
) after having undergone high mass-accretion rates ( ). Their spin
distribution upon arrival at the zero age main sequence (ZAMS) is consistent
with that observed for young B stars, assuming conservation of angular momentum
and homologous contraction.Comment: Accepted for publication in A&A. Appendixes A and B have been
truncated due to size limitations, the full version will be available on A&
Corona, Jet, and Relativistic Line Models for Suzaku/RXTE/Chandra-HETG Observations of the Cygnus X-1 Hard State
Using Suzaku and the Rossi X-ray Timing Explorer, we have conducted a series
of four simultaneous observations of the galactic black hole candidate Cyg X-1
in what were historically faint and spectrally hard low states. Additionally,
all of these observations occurred near superior conjunction with our line of
sight to the X-ray source passing through the dense phases of the focused wind
from the mass donating secondary. One of our observations was also simultaneous
with observations by the Chandra-High Energy Transmission Grating. These latter
spectra are crucial for revealing the ionized absorption due to the secondary's
focused wind. Such absorption is present and must be accounted for in all four
spectra. These simultaneous data give an unprecedented view of the 0.8-300 keV
spectrum of Cyg X-1, and hence bear upon both corona and X-ray emitting jet
models of black hole hard states. Three models fit the spectra well: coronae
with thermal or mixed thermal/non-thermal electron populations, and jets. All
three models require a soft component that we fit with a low temperature disk
spectrum with an inner radius of only a few tens of GM/c^2. All three models
also agree that the known spectral break at 10\,keV is not solely due to the
presence of reflection, but each gives a different underlying explanation for
the augmentation of this break. Thus whereas all three models require that
there is a relativistically broadened Fe line, the strength and inner radius of
such a line is dependent upon the specific model, {thus making premature
line-based estimates of the black hole spin in the Cyg X-1 system. We look at
the relativistic line in detail, accounting for the narrow Fe emission and
ionized absorption detected by HETG. Although the specific relativistic
parameters of the line are continuum-dependent, none of the broad line fits
allow for an inner disk radius that is >40 GM/c^2.Comment: 22 pages, 16 figures. Uses emulateapj style. Final three tables
inserted as a figure to avoid issues with astro-ph's version of latex
mangling the use of lscape. To be published in the Astrophysical Journal,
January, 201
Reconstructing EUV spectrum of star forming regions from millimeter recombination lines of HI, HeI, and HeII
The extreme ultraviolet (EUV) spectra of distant star-forming regions cannot
be probed directly using either ground- or space-based telescopes due to the
high cross-section for interaction of EUV photons with the interstellar medium.
This makes EUV spectra poorly constrained. The mm/submm recombination lines of
H and He, which can be observed from the ground, can serve as a reliable probe
of the EUV. Here we present a study based on ALMA observations of three
Galactic ultra-compact HII regions and the starburst region Sgr B2(M), in which
we reconstruct the key parameters of the EUV spectra using mm recombination
lines of HI, HeI and HeII. We find that in all cases the EUV spectra between
13.6 and 54.4 eV have similar frequency dependence: L_{\nu}~ \nu^{-4.5 +/-
0.4}. We compare the inferred values of the EUV spectral slopes with the values
expected for a purely single stellar evolution model (Starburst99) and the
Binary Population and Spectral Synthesis code (BPASS). We find that the
observed spectral slope differs from the model predictions. This may imply that
the fraction of interacting binaries in HII regions is substantially lower than
assumed in BPASS. The technique demonstrated here allows one to deduce the EUV
spectra of star forming regions providing critical insight into photon
production rates at \lambda < 912 A and can serve as calibration to starburst
synthesis models, improving our understanding of star formation in distant
universe and the properties of ionizing flux during reionization.Comment: v1: 18 pages, 5 figures, 1 table, 1 appendix v2: BPASS version 2.1 ->
2.2.1, SB99 tracks explicitly added to figures, evolution with metallicity
added, few points clarified. 20 pages, 6 figures, 1 table. Accepted to Ap
Reconstructing the EUV Spectrum of Star-forming Regions from Millimeter Recombination Lines of H I, He I, and He II
The extreme ultraviolet (EUV) spectra of distant star-forming regions cannot be probed directly using either ground- or space-based telescopes due to the high cross section for interaction of EUV photons with the interstellar medium. This makes EUV spectra poorly constrained. The millimeter/submillimeter recombination lines of H and He, which can be observed from the ground, can serve as a reliable probe of the EUV. Here we present a study based on ALMA observations of three Galactic ultracompact H II regions and the starburst region Sgr B2(M), in which we reconstruct the key parameters of the EUV spectra using millimeter recombination lines of H I, He I, and He II. We find that in all cases the EUV spectra between 13.6 and 54.4 eV have similar frequency dependence: L_ ν ∝ ν^(-4.5±0.4). We compare the inferred values of the EUV spectral slopes with the values expected for a purely single stellar evolution model (Starburst99) and the Binary Population and Spectral Synthesis code (BPASS). We find that the observed spectral slope differs from the model predictions. This may imply that the fraction of interacting binaries in H II regions is substantially lower than assumed in BPASS. The technique demonstrated here allows one to deduce the EUV spectra of star-forming regions, providing critical insight into photon production rates at λ ⩽ 912 Å, and can serve as calibration to starburst synthesis models, improving our understanding of star formation in the distant universe and the properties of ionizing flux during reionization
The young massive SMC cluster NGC 330 seen by MUSE - II. Multiplicity properties of the massive-star population
Context. Observations of massive stars in open clusters younger than ∼8 Myr have shown that a majority of them are in binary systems, most of which will interact during their life. While these can be used as a proxy of the initial multiplicity properties, studying populations of massive stars older than ∼20 Myr allows us to probe the outcome of these interactions after a significant number of systems have experienced mass and angular momentum transfer and may even have merged. Aims. Using multi-epoch integral-field spectroscopy, we aim to investigate the multiplicity properties of the massive-star population in the dense core of the ∼40 Myr old cluster NGC 330 in the Small Magellanic Cloud in order to search for possible imprints of stellar evolution on the multiplicity properties. Methods. We obtained six epochs of VLT/MUSE observations operated in wide-field mode with the extended wavelength setup and supported by adaptive optics. We extracted spectra and measured radial velocities for stars brighter than mF814W = 19. We identified single-lined spectroscopic binaries through significant RV variability with a peak-to-peak amplitude larger than 20 km s−1. We also identified double-lined spectroscopic binaries, and quantified the observational biases for binary detection. In particular, we took into account that binary systems with similar line strengths are difficult to detect in our data set. Results. The observed spectroscopic binary fraction among stars brighter than mF814W = 19 (approximately 5.5 M⊙ on the main sequence) is fSBobs = 13.2 ± 2.0%. Considering period and mass ratio ranges from log(P) = 0.15−3.5 (about 1.4 to 3160 d), q = 0.1−1.0, and a representative set of orbital parameter distributions, we find a bias-corrected close binary fraction of fcl = 34−7+8%. This fraction seems to decline for the fainter stars, which indicates either that the close binary fraction drops in the B-type domain, or that the period distribution becomes more heavily weighted toward longer orbital periods. We further find that both fractions vary strongly in different regions of the color-magnitude diagram, which corresponds to different evolutionary stages. This probably reveals the imprint of the binary history of different groups of stars. In particular, we find that the observed spectroscopic binary fraction of Be stars (fSBobs = 2 ± 2%) is significantly lower than that of B-type stars (fSBobs = 9 ± 2%). Conclusions. We provide the first homogeneous radial velocity study of a large sample of B-type stars at a low metallicity ([Fe/H] ≲ −1.0). The overall bias-corrected close binary fraction (log(P) < 3.5 d) of the B-star population in NGC 330 is lower than the fraction reported for younger Galactic and Large Magellanic Cloud clusters in previous works. More data are needed, however, to establish whether the observed differences are caused by an age or a metallicity effect.The authors acknowledge support from the FWO_Odysseus program under project G0F8H6N and from the European Space Agency (ESA) and the Belgian Federal Science Policy Office (BELSPO) through the PRODEX Programme. The research leading to these results has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement numbers 772225: MULTIPLES). Parts of the analysis in this project are based on the python code photutils
Evolved Massive Stars at Low-metallicity V. Mass-Loss Rate of Red Supergiant Stars in the Small Magellanic Cloud
We assemble the most complete and clean red supergiant (RSG) sample (2,121
targets) so far in the Small Magellanic Cloud (SMC) with 53 different bands of
data to study the MLR of RSGs. In order to match the observed spectral energy
distributions (SEDs), a theoretical grid of 17,820 Oxygen-rich models
(``normal'' and ``dusty'' grids are half-and-half) is created by the
radiatively-driven wind model of the DUSTY code, covering a wide range of dust
parameters. We select the best model for each target by calculating the minimal
modified chi-square and visual inspection. The resulting MLRs from DUSTY are
converted to real MLRs based on the scaling relation, for which a total MLR of
yr is measured (corresponding to a
dust-production rate of yr), with a
typical MLR of yr for the general population of
the RSGs. The complexity of mass-loss estimation based on the SED is fully
discussed for the first time, indicating large uncertainties based on the
photometric data (potentially up to one order of magnitude or more). The
Hertzsprung-Russell and luminosity versus median absolute deviation diagrams of
the sample indicate the positive relation between luminosity and MLR.
Meanwhile, the luminosity versus MLR diagrams show a ``knee-like'' shape with
enhanced mass-loss occurring above , which may
be due to the degeneracy of luminosity, pulsation, low surface gravity,
convection, and other factors. We derive our MLR relation by using a
third-order polynomial to fit the sample and compare our result with previous
empirical MLR prescriptions. Given that our MLR prescription is based on a much
larger sample than previous determinations, it provides a more accurate
relation at the cool and luminous region of the H-R diagram at low-metallicity
compared to previous studies.Comment: 16 pages, 19 figures, accepted by A&
Reconstructing the EUV Spectrum of Star-forming Regions from Millimeter Recombination Lines of HI, HeI, and HeII
The extreme ultraviolet (EUV) spectra of distant star-forming regions cannot be probed directly using either ground- or space-based telescopes due to the high cross section for interaction of EUV photons with the interstellar medium. This makes EUV spectra poorly constrained. The millimeter/submillimeter recombination lines of H and He, which can be observed from the ground, can serve as a reliable probe of the EUV. Here we present a study based on ALMA observations of three Galactic ultracompact H II regions and the starburst region Sgr B2(M), in which we reconstruct the key parameters of the EUV spectra using millimeter recombination lines of H I, He I, and He II. We find that in all cases the EUV spectra between 13.6 and 54.4 eV have similar frequency dependence: Ln n-4.5+0.4. We compare the inferred values of the EUV spectral slopes with the values expected for a purely single stellar evolution model (Starburst99) and the Binary Population and Spectral Synthesis code (BPASS). We find that the observed spectral slope differs from the model predictions. This may imply that the fraction of interacting binaries in H II regions is substantially lower than assumed in BPASS. The technique demonstrated here allows one to deduce the EUV spectra of star-forming regions, providing critical insight into photon production rates at l * 912A, and can serve as calibration to starburst synthesis models, improving our understanding of star formation in the distant universe and the properties of ionizing flux during reionization.L.M.’s stipend at the IAS is provided by the Friends of the
Institute for Advanced Study. A part of this work was
conducted while L.M. was supported by the SOS NRAO
program and as a Groce Fellow at Caltech. L.M. is grateful to
Dr. David and Barbara Groce for their kindness and support.
Part of this research was carried out at the Jet Propulsion
Laboratory, California Institute of Technology, under a contract
with the National Aeronautics and Space Administratio