Context.
Spectroscopic multiplicity surveys of O stars in young clusters and OB associations have revealed that a large portion (∼70%) of these massive stars (
M
i
> 15
M
⊙
) belong to close and ...short-period binaries (with a physical separation of less than a few astronomical units). Follow-up VLT(I) high-angular-resolution observations led to the detection of wider companions (up to
d
∼ 500 au), increasing the average companion fraction to > 2. Despite the recent and significant progress, the formation mechanisms leading to such close massive multiple systems remain to be elucidated. As a result, young massive close binaries (or higher-order multiple systems) are unique laboratories for determining the pairing mechanism of high-mass stars.
Aims.
We present the first VLTI/GRAVITY observations of six young O stars in the M17 star-forming region (≲1 Myr) and two additional foreground stars. VLTI/GRAVITY provides the
K
-band high-angular-resolution observations needed to explore the close environment of young O-type stars, and, as such, offers an excellent opportunity to characterise the multiplicity properties of the immediate outcome of the massive star formation process.
Methods.
From the interferometric model fitting of visibility amplitudes and closure phases, we search for companions and measure their positions and flux ratios. Combining the resulting magnitude difference with atmosphere models and evolutionary tracks, we further constrain the masses of the individual components.
Results.
All six high-mass stars are in multiple systems, leading to a multiplicity fraction of 100% and yielding a 68% confidence interval of 94–100%. We detect a total of nine companions with separations of up to 120 au. Including previously identified spectroscopic companions, the companion fraction of the young O stars in our sample reaches 2.3 ± 0.6. The derived masses span a wide range, from 2.5 to 50
M
⊙
, with a great tendency towards high-mass companions. However, we do not find a significant correlation between the mass of the companions and their separation.
Conclusions.
While based on a modest sample, our results clearly indicate that the origin of the high degree of multiplicity is rooted in the star formation mechanism of the sample stars. No clear evidence for one of the competing concepts of massive star formation (core accretion or competitive accretion) could be found. However, given that we find all of the companions within ∼120 au, our results are compatible with migration as a scenario for the formation of close massive binaries.
Context.
Characterising the outcome of the star formation process is key to understand and predict the evolution of stellar populations. Especially the fraction of massive stars in young stellar ...clusters is of importance as they are the dominant sources of both mechanical and radiative feedback, strongly influencing the thermal and dynamical state of their birth environments, and beyond. Their supernovae may trigger the formation of new generations of stars in neighbouring regions. It turns out that a significant fraction of massive stars escape from their parent cluster via dynamical interactions of single stars and/or multiple stellar systems.
Aims.
M 17 is the nearest giant H
II
region hosting a very young and massive cluster: NGC 6618. Our aim is to identify stars brighter than
G
≲ 21 mag that belong to NGC 6618, including the (massive) stars that may have escaped since its formation, and to determine the cluster distance and age.
Methods.
The
Gaia
DR3 database was used to identify members of NGC 6618 based on parallax and proper motion within 9′ from the cluster centre. We searched for nearby stars in a field of 5° around the cluster centre that may have originated from the cluster, and we determined their transverse velocity, kinematic age, and impact parameter.
Results.
We identified 42 members of NGC 6618 of which eight have a spectral type of O, with a mean distance of 1675
−18
+19
pc and a (transversal) velocity dispersion of about 3 km s
−1
, and a radial velocity dispersion of ∼6 km s
−1
. Another ten O stars are associated with NGC 6618, but they cannot be classified as members due to poor astrometry and/or high extinction. We have also identified six O star runaways. The relative transverse velocity of these runaways ranges from 10 to 70 km s
−1
and their kinematic age ranges from about 100 to 750 kyr. Given the already established young age of NGC 6618 (≲1 Myr), this implies that massive stars are being ejected from the cluster already directly after (or during) the cluster formation process.
Conclusions.
When constructing the initial mass function, one has to take into account the massive stars that have already escaped from the cluster, that is, about 30% of the O stars of the original population of NGC 6618. The trajectories of the O runaways can be traced back to the central 0.2–0.3 pc region of NGC 6618. The good agreement between the evolutionary and kinematic age of the runaways implies that the latter provides an independent way to estimate (a lower limit to) the age of the cluster.
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 (
σ
1D
) 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
f
bin
= 0.7, we converted the
σ
1D
behavior to an evolution of the minimum orbital period
P
cutoff
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.
Context
. It is a challenge to study the formation process of massive stars: their formation time is short, there are only few of them, they are often deeply embedded, and they lie at relatively ...large distances. Our strategy is to study the outcome of the star formation process and to search for signatures that remain of the formation. We have access to a unique sample of (massive) pre-main-sequence (PMS) stars in the giant H
II
region M17. These PMS stars can be placed on PMS tracks in the Hertzsprung–Russell diagram (HRD) as we can detect their photospheric spectrum, and they exhibit spectral features indicative of the presence of a circumstellar disk. These stars are most likely in the final stage of formation.
Aims
. The aim is to use spectroscopic variability as a diagnostic tool to learn about the physical nature of these massive PMS stars. More specifically, we wish to determine the variability properties of the hot gaseous disks to understand the physical origin of the emission lines, identify dominant physical processes in these disks, and to find out about the presence of an accretion flow and/or jet.
Methods
. We obtained multiple-epoch (four to five epochs) VLT/X-shooter spectra of six young stars in M17 covering about a decade. Four of these stars are intermediate to massive PMS stars with gaseous disks. Using stacked spectra, we updated the spectral classification and searched for the presence of circumstellar features. With the temporal variance method (TVS), we determined the extent and amplitude of the spectral line variations in velocity space. The double-peaked emission lines in the PMS stars with gaseous disks were used to determine peak-to-peak velocities, V/R ratios, and the radial velocity of the systems. Simultaneous photometric variations were studied using VLT acquisition images.
Results
. From detailed line identification in the PMS stars with gaseous disks, we identify many (double-peaked) disk features, including a new detection of CO bandhead and C
I
emission. In three of these stars, we detect significant spectral variability, mainly in lines originating in the circumstellar disk, in a velocity range up to 320 km s
−1
, which exceeds the rotational velocity of the central sources. The shortest variability timescale is about one day. We also detect long-term (months, years) variability. The ratio of the blue and red peaks in two PMS stars shows a correlation with the peak-to-peak velocity, which might be explained by a spiral-arm structure in the disk.
Conclusions
. The variable PMS stars lie 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 line (sets), showing the complexity of the region where the lines are formed. We find indications for an accretion flow, slow disk winds, and/or disk structures in the hot gaseous inner disks. We find no evidence for close companions or strong accretion bursts as the cause of the variability in these PMS stars.
Massive pre-main-sequence stars in M17 Poorta, J.; Ramírez-Tannus, M. C.; de Koter, A. ...
Astronomy and astrophysics (Berlin),
08/2023, Letnik:
676
Journal Article
Recenzirano
Odprti dostop
Context.
Recently much progress has been made in probing the embedded stages of massive star formation, pointing to formation scenarios that are reminiscent of a scaled-up version of low-mass star ...formation. However, the latest stages of massive-star formation have rarely been observed, as young massive stars are assumed to reveal their photospheres only when they are fully formed.
Aims.
Using first and second overtone CO bandhead emission and near- to mid-infrared photometry, we aim to characterize the remnant formation disks around five unique pre-main-sequence (PMS) stars with masses 6–12
M
⊙
that have constrained stellar parameters thanks to their detectable photospheres. We seek to understand this emission and the disks from which it originates in the context of the evolutionary stage of the studied sources.
Methods.
We used an analytic disk model, and adopted local thermodynamical equilibrium, to fit the CO bandhead and the dust emission, assumed to originate in different disk regions. For the first time, we modeled the second overtone emission, which helped us to put tighter constraints on the density of the CO gas. Furthermore, we fit continuum normalized bandheads, using models for stellar and dust continuum, and show the importance of this in constraining the emission region. We also included
13
CO in our models as an additional probe of the young nature of the studied objects.
Results.
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 and identifies CO emission as an indicator of the presence of a gaseous inner disk reaching close to the stellar surface. From constraining the location of the inner edge of the dust emission, we find that all but one of the objects have undisrupted inner dust disks.
Conclusions.
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 an early age and high accretion rates in (massive) young stellar objects. We conclude, considering their mass range and the fact that their photospheres are detected, that 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.
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 P
RO
D
I
M
O
of double-peaked hydrogen lines of the Paschen and Brackett series observed with X-shooter was used to probe the properties of the inner disk of the target sources. The model was modified to treat these lines. Additionally, the dust structure was studied by fitting the optical and near-infrared spectral energy distribution.
Results.
B243 features a hot gaseous inner disk with dust at the sublimation radius at ~3 AU. The disk appears truncated at roughly 6.5 AU; a cool outer disk of gas and dust may be present, but it cannot be detected with our data. B331 also has a hot gaseous inner disk. A gap separates the inner disk from a colder dusty outer disk starting at up to ~100 AU. In both sources the inner disk extends to almost the stellar surface. Chemistry is essential for the ionization of hydrogen in these disks.
Conclusions.
The lack of a gap between the central objects and these disks suggests that they accrete through boundary-layer accretion. This would exclude the stars having a strong magnetic field. Their structures suggest that both disks are transitional in nature, that is to say they are in the process of being cleared, either through boundary-layer accretion, photo-evaporation, or through companion activity.
Context.
Diffuse interstellar bands (DIBs) are broad absorption features measured in sightlines probing the diffuse interstellar medium. Although large carbon-bearing molecules have been proposed as ...the carriers producing DIBs, their identity remains unknown. DIBs make an important contribution to the extinction curve; the sightline to the young massive star-forming region M17 shows anomalous extinction in the sense that the total-to-selective extinction parameter (
R
V
) differs significantly from the average Galactic value and may reach values
R
V
> 4. Anomalous DIBs have been reported in the sightline towards Herschel 36 (
R
V
= 5.5), in the massive star-forming region M8. Higher values of
R
V
have been associated with a relatively higher fraction of large dust grains in the line of sight.
Aims.
Given the high
R
V
values, we investigate whether the DIBs in sightlines towards young OB stars in M17 show a peculiar behaviour.
Methods.
We measure the properties of the most prominent DIBs in M17 and study these as a function of
E
(
B
–
V
) and
R
V
. We also analyse the gaseous and dust components contributing to the interstellar extinction.
Results.
The DIB strengths in M17 concur with the observed relations between DIB equivalent width and reddening
E
(
B
–
V
) in Galactic sightlines. For several DIBs we discover a linear relation between the normalised DIB strength EW/
A
V
and
R
V
−1
. These trends suggest two groups of DIBs: (i) a group of ten moderately strong DIBs that show a sensitivity to changes in
R
V
that is modest and proportional to DIB strength, and (ii) a group of four very strong DIBs that react sensitively and to a similar degree to changes in
R
V
, but in a way that does not appear to depend on DIB strength.
Conclusions.
DIB behaviour as a function of reddening is not peculiar in sightlines to M17. Also, we do not detect anomalous DIB profiles like those seen in Herschel 36. DIBs are stronger, per unit visual extinction, in sightlines characterised by a lower value of
R
V
, i.e. those sightlines that contain a relatively large fraction of small dust particles. New relations between extinction normalised DIB strengths, EW/
A
V
, and
R
V
support the idea that DIB carriers and interstellar dust are intimately connected. Furthermore, given the distinct behaviour of two groups of DIBs, different types of carriers do not necessarily relate to the dust grains in a similar way.
The process of massive star (
M
≥ 8
M
⊙
) formation is still poorly understood. Observations of massive young stellar objects (MYSOs) are challenging due to their rarity, short formation timescale, ...large distances, and high circumstellar extinction. Here, we present the results of a spectroscopic analysis of a population of MYSOs in the Large Magellanic Cloud. We took advantage of the spectral resolution and wavelength coverage of X-shooter (300−2500 nm), which is mounted on the European Southern Observatory Very Large Telescope, to detect characteristic spectral features in a dozen MYSO candidates near 30 Doradus, the largest starburst region in the Local Group hosting the most massive stars known. The X-shooter spectra are strongly contaminated by nebular emission. We used a scaling method to subtract the nebular contamination from our objects. We detect H
α
,
β
, O
I
630.0 nm, Ca
II
, infrared triplet Fe
II
1643.5 nm, fluorescent Fe
II
1687.8 nm, H
2
2121.8 nm, Br
γ
, and CO bandhead emission in the spectra of multiple candidates. This leads to the spectroscopic confirmation of ten candidates as bona fide MYSOs. We compared our observations with photometric observations from the literature and find all MYSOs to have a strong near-infrared excess. We computed lower limits to the brightness and luminosity of the MYSO candidates, confirming the near-infrared excess and the massive nature of the objects. No clear correlation is seen between the Br
γ
luminosity and metallicity. Combining our sample with other LMC samples results in a combined detection rate of disk features, such as fluorescent Fe
II
and CO bandheads, which is consistent with the Galactic rate (40%). Most of our MYSOs show outflow features.
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
≈ 0.5 − 5
M
⊙
). The ages of the clusters are ∼1−3 Myr, < 3 Myr, and ∼0.5−3 Myr for M 8, G333.6−0.2, and NGC 6357, respectively. We show that MYStIX selected targets have > 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%.
Context. Diffuse interstellar bands (DIBs) are broad absorption features measured in sightlines probing the diffuse interstellar medium. Although large carbon-bearing molecules have been proposed as ...the carriers producing DIBs, their identity remains unknown. DIBs make an important contribution to the extinction curve; the sightline to the young massive star-forming region M17 shows anomalous extinction in the sense that the total-to-selective extinction parameter (RV) differs significantly from the average Galactic value and may reach values RV > 4. Anomalous DIBs have been reported in the sightline towards Herschel 36 (RV = 5.5), in the massive star-forming region M8. Higher values of RV have been associated with a relatively higher fraction of large dust grains in the line of sight. Aims. Given the high RV values, we investigate whether the DIBs in sightlines towards young OB stars in M17 show a peculiar behaviour. Methods. We measure the properties of the most prominent DIBs in M17 and study these as a function of E(B–V) and RV. We also analyse the gaseous and dust components contributing to the interstellar extinction. Results. The DIB strengths in M17 concur with the observed relations between DIB equivalent width and reddening E(B–V) in Galactic sightlines. For several DIBs we discover a linear relation between the normalised DIB strength EW/AV and RV−1 $R_{V}^{-1}$ R V −1 . These trends suggest two groups of DIBs: (i) a group of ten moderately strong DIBs that show a sensitivity to changes in RV that is modest and proportional to DIB strength, and (ii) a group of four very strong DIBs that react sensitively and to a similar degree to changes in RV, but in a way that does not appear to depend on DIB strength. Conclusions. DIB behaviour as a function of reddening is not peculiar in sightlines to M17. Also, we do not detect anomalous DIB profiles like those seen in Herschel 36. DIBs are stronger, per unit visual extinction, in sightlines characterised by a lower value of RV, i.e. those sightlines that contain a relatively large fraction of small dust particles. New relations between extinction normalised DIB strengths, EW/AV, and RV support the idea that DIB carriers and interstellar dust are intimately connected. Furthermore, given the distinct behaviour of two groups of DIBs, different types of carriers do not necessarily relate to the dust grains in a similar way.
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