7 research outputs found
Massive pre-main-sequence stars in M17: and overtone CO bandhead emission and the thermal infrared
Recently much progress has been made in probing the embedded stages of
massive star formation, pointing to formation scenarios akin to a scaled up
version of low-mass star formation. However, the latest stages of massive star
formation have rarely been observed. Using 1st and 2nd overtone CO bandhead
emission and near- to mid-infrared photometry we aim to characterize the
remnant formation disks around 5 unique pre-main-sequence (PMS) stars with
masses , that have constrained stellar parameters thanks to
their detectable photospheres. We seek to understand this emission and the
disks it originates from in the context of the evolutionary stage of the
studied sources. We use an analytic LTE disk model to fit the CO bandhead and
the dust emission, found to originate in different disk regions. For the first
time we modeled the 2nd overtone emission. Furthermore, we fit continuum
normalized bandheads and show the importance of this in constraining the
emission region. We also include in our models as an additional
probe of the young nature of the studied objects. We find that the CO emission
originates in a narrow region close to the star (<1 AU) and under very similar
disk conditions (temperatures and densities) for the different objects. This is
consistent with previous modeling of this emission in a diverse range of young
stellar objects. We discuss these results in the context of the positions of
these PMS stars in the Hertzsprung-Russel diagram and the CO emission's
association with early age and high accretion rates in (massive) young stellar
objects. We conclude that, considering their mass range and for the fact that
their photospheres are detected, the M17 PMS stars are observed in a relatively
early formation stage. They are therefore excellent candidates for longer
wavelength studies to further constrain the end stages of massive star
formation.Comment: 21 pages, 12 figure
Spectroscopic variability of massive pre-main-sequence stars in M17
It is a challenge to study the formation process of massive stars: their
formation time is short, they are few, often deeply embedded, and at relatively
large distances. Our strategy is to study the outcome of the star formation
process and to look for signatures remnant of the formation. We have access to
a unique sample of (massive) pre-main-sequence (PMS) stars in the giant HII
region M17, showing a photosphere and circumstellar disk. The aim is to
determine the variability properties of the hot gaseous disks to understand the
physical origin of the emission lines and identify dominant physical processes
in these disks. We have obtained multiple-epoch (4-5 epochs) VLT/X-shooter
spectra of six young stars in M17 covering about a decade. Using stacked
spectra we update the spectral classification and identify circumstellar
features. With the temporal variance method (TVS) we determine the extent and
amplitude of the spectral line variations. The double-peaked emission lines in
the PMS stars with gaseous disks are used to determine peak-to-peak velocities,
V/R-ratios and the radial velocity of the systems. We identify many disk
features, under which a new detection of CO bandhead and CI emission. In three
of the stars we detect spectral variability, mainly in lines originating in the
circumstellar disk, in a velocity range up to 320 km/s. In two PMS stars the
ratio between the blue and red peaks shows a correlation with the peak-to-peak
velocity, possibly explained by a spiral-arm structure in the disk. The PMS
stars with variability are at similar positions in the HRD but show significant
differences in disk lines and variability. The extent and timescale of the
variability differs for each star and per line (sets). We find indications for
an accretion flow, slow disk winds and/or disk structures in the hot gaseous
inner disk as the cause of the variability in these PMS stars.Comment: 27 pages, 24 figures, accepted for publication in Astronomy and
Astrophysics, abstract abbreviate
The young stellar content of the giant H II regions M 8, G333.6-0.2, and NGC 6357 with VLT/KMOS
Context. The identification and characterisation of populations of young massive stars in (giant) H II regions provides important constraints on (i) the formation process of massive stars and their early feedback on the environment, and (ii) the initial conditions for population synthesis models predicting the evolution of ensembles of stars. Aims. We identify and characterise the stellar populations of the following young giant H II regions: M 8, G333.6-0.2, and NGC 6357. Methods. We have acquired H-and K-band spectra of around 200 stars using the K-band Multi Object Spectrograph on the ESO Very Large Telescope. The targets for M 8 and NGC 6357 were selected from the Massive Young Star-Forming Complex Study in Infrared and X-ray (MYStIX), which combines X-ray observations with near-infrared (NIR) and mid-infrared data. For G333.6-0.2, the sample selection is based on the NIR colours combined with X-ray data. We introduce an automatic spectral classification method in order to obtain temperatures and luminosities for the observed stars. We analysed the stellar populations using their photometric, astrometric, and spectroscopic properties and compared the position of the stars in the Hertzprung-Russell diagram with stellar evolution models to constrain their ages and mass ranges. Results. We confirm the presence of candidate ionising sources in the three regions and report new ones, including the first spectroscopically identified O stars in G333.6-0.2. In M 8 and NGC 6357, two populations are identified: (i) OB main-sequence stars (M > 5 M· ) and (ii) pre-main sequence stars (M ≈ 0.5ℓ-ℓ 5 M· ). The ages of the clusters are ∼1-3 Myr, 90% probability of being members of the H II region, whereas a selection based on NIR colours leads to a membership probability of only ∼70%
Massive pre-main-sequence stars in M17
Context. The young massive-star-forming region M17 contains optically visible massive pre-main-sequence stars that are surrounded by circumstellar disks. Such disks are expected to disappear when these stars enter the main sequence. The physical and dynamical structure of these remnant disks are poorly constrained, especially the inner regions where accretion, photo-evaporation, and companion formation and migration may be ongoing.
Aims. We aim to constrain the physical properties of the inner parts of the circumstellar disks of massive young stellar objects B243 (6 M⊙) and B331 (12 M⊙), two systems for which the central star has been detected and characterized previously despite strong dust extinction.
Methods. Two-dimensional radiation thermo-chemical modelling with PR
A relation between the radial velocity dispersion of young clusters and their age
The majority of massive stars (> 8 M⊙) in OB associations are found in close binary systems. Nonetheless, the formation mechanism of these close massive binaries is not understood yet. Using literature data, we measured the radial-velocity dispersion (σ1D) as a proxy for the close binary fraction in ten OB associations in the Galaxy and the Large Magellanic Cloud, spanning an age range from 1 to 6 Myr. We find a positive trend of this dispersion with the cluster’s age, which is consistent with binary hardening. Assuming a universal binary fraction of fbin = 0.7, we converted the σ1D behavior to an evolution of the minimum orbital period Pcutoff from ∼9.5 years at 1 Myr to ∼1.4 days for the oldest clusters in our sample at ∼6 Myr. Our results suggest that binaries are formed at larger separations, and they harden in around 1 to 2 Myr to produce the period distribution observed in few million year-old OB binaries. Such an inward migration may either be driven by an interaction with a remnant accretion disk or with other young stellar objects present in the system. Our findings constitute the first empirical evidence in favor of migration as a scenario for the formation of massive close binaries