Diffuse interstellar bands in the HII region M17: Insights into their relation with the total-to-selective visual extinction RVR_V

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. The sight line to the young massive star-forming region M17 shows anomalous extinction in the sense that the total-to-selective extinction parameter differs significantly from the average Galactic value and may reach values $R_{V} > 4$. Given the high $R_V$ values, we investigate whether the DIBs in sight lines towards young OB stars in M17 show a peculiar behaviour. We measure the properties of the most prominent DIBs in M17 and study these as a function of $E(B-V)$ and $R_{V}$. The DIB strengths in M17 concur with the observed relations between DIB equivalent width and reddening $E(B-V)$ in Galactic sight lines. 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: (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. The DIB behaviour as a function of reddening is not peculiar in sight lines to M17. Also, we do not detect anomalous DIB profiles as seen in Her 36. DIBs are stronger, per unit visual extinction, in sight lines characterised by a smaller value of $R_{V}$ (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 connected. 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.Comment: Abstract has been shortened. Accepted for publication in A&A. 14 pages, 7 pages of appendix, 28 figure

Massive pre-main-sequence stars in M17: 1st1^{\rm st} and 2nd2^{\rm nd} 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 $6-12~\rm M_{\odot}$, 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 $^{13}\rm CO$ 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

X-Shooting ULLYSES: Massive stars at low metallicity: I. Project description

Observations of individual massive stars, super-luminous supernovae, gamma-ray bursts, and gravitational wave events involving spectacular black hole mergers indicate that the low-metallicity Universe is fundamentally different from our own Galaxy. Many transient phenomena will remain enigmatic until we achieve a firm understanding of the physics and evolution of massive stars at low metallicity (Z). The Hubble Space Telescope has devoted 500 orbits to observing ∼250 massive stars at low Z in the ultraviolet (UV) with the COS and STIS spectrographs under the ULLYSES programme. The complementary X-Shooting ULLYSES (XShootU) project provides an enhanced legacy value with high-quality optical and near-infrared spectra obtained with the wide-wavelength coverage X-shooter spectrograph at ESOa's Very Large Telescope. We present an overview of the XShootU project, showing that combining ULLYSES UV and XShootU optical spectra is critical for the uniform determination of stellar parameters such as effective temperature, surface gravity, luminosity, and abundances, as well as wind properties such as mass-loss rates as a function of Z. As uncertainties in stellar and wind parameters percolate into many adjacent areas of astrophysics, the data and modelling of the XShootU project is expected to be a game changer for our physical understanding of massive stars at low Z. To be able to confidently interpret James Webb Space Telescope spectra of the first stellar generations, the individual spectra of low-Z stars need to be understood, which is exactly where XShootU can deliver

On the origin of close massive binaries in the M17 star-forming region

Spectroscopic multiplicity surveys of O stars in young clusters and OB associations have revealed that a large portion ($\sim$ 70%) of these massive stars (M$_{i}$ $\gt$ 15 $M_{\odot}$) belong to close and short-period binaries (physical separation d $\lt$few au). 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 to figure out the pairing mechanism of high-mass stars. We present the first VLTI/GRAVITY observations of six young O stars in the M17 star-forming region ($\lesssim$ 1 Myr) and two additional foreground stars. 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. All of the six high-mass stars are in multiple systems, leading to a multiplicity fraction (MF) of 100%, yielding a 68% confidence interval of 94-100%. We detect a total number of 9 companions with separations up to 120 au. Including previously identified spectroscopic companions, the companion fraction of the young O stars in our sample reaches 2.3$\pm$0.6. The derived masses span a wide range from 2.5 to 50 $M_{\odot}$, with a great tendency towards high-mass companions. While based on a modest sample, our results clearly indicate that the origin of the high degree of multiplicity is rooted in their star formation mechanism. No clear evidence for one of the competing concepts of massive star formation (core accretion or competitive accretion) could be found. However, our results are compatible with migration as a scenario for the formation of close massive binaries.Comment: 20 pages, 14 figures, 4 tables, 3 appendice

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