A monolithic collapse origin for the thin/thick disc structure of ESO 243-49

ESO 243-49 is a high-mass (circular velocity $v_{\rm c}\approx200\,{\rm km\,s^{-1}}$) edge-on S0 galaxy in the Abell 2877 cluster at a distance of $\sim95\,{\rm Mpc}$. To elucidate the origin of its thick disc, we use MUSE science verification data to study its kinematics and stellar populations. The thick disc emits $\sim80\%$ of the light at heights in excess of $3.5^{\prime\prime}$ ($1.6\,{\rm kpc}$). The rotation velocities of its stars lag by $30-40\,{\rm km\,s^{-1}}$ compared to those in the thin disc, which is compatible with the asymmetric drift. The thick disc is found to be more metal-poor than the thin disc, but both discs have old ages. We suggest an internal origin for the thick disc stars in high-mass galaxies. We propose that the thick disc formed either ${\rm a)}$ first in a turbulent phase with a high star formation rate and that a thin disc formed shortly afterwards, or ${\rm b)}$ because of the dynamical heating of a thin pre-existing component. Either way, the star formation in ESO 243-49 was quenched just a few Gyrs after the galaxy was born and the formation of a thin and a thick disc must have occurred before the galaxy stopped forming stars. The formation of the discs was so fast that it could be described as a monolithic collapse where several generations of stars formed in a rapid succession.Comment: Accepted for publication in A&A. The reduced data-cube as well as the data necessary to build the kinematic and stellar population maps are available at https://etsin.avointiede.fi/dataset/urn-nbn-fi-csc-kata2016092414291163237

The K supergiant runaway star HD 137071

Very few examples are known of red supergiant runaways, all of them descending from the more massive O-type precursors, but none from the lower mass B-type precursors, although runaway statistics among B-type stars suggest that K-type runaways must be relatively numerous. We study HD 137071, a star that has been considered so far as a normal K-type red giant. Its parallax measured by Gaia and the derived luminosity suggest that it is actually a supergiant, whereas its derived distance to the galactic plane and its spatial velocity of 54.1 km s$^{-1}$ with respect to the local standard of rest suggest that it is also a runaway star. However, intrinsic limitations in determining the trigonometric parallaxes of cool supergiants, even in the Gaia era, require accurate spectral classifications for confirmation. We reliably classify HD 137071 as a K4II star establishing its membership to the extreme Population I, which is in agreement with the luminosity derived using the Gaia DR2 parallax measurement. Kinematical data from the Gaia DR2 catalog confirm its high spatial velocity and its runaway nature. Combining the spectral classification with astrometric information, a state-of-the-art galactic potential model, and evolutionary models for high-mass stars we trace the motion of HD 137071 back to the proximities of the galactic plane and speculate on which of the two proposed mechanisms for the production of runaway stars may be responsible for the high velocity of HD 137071. The available data favor the formation of HD 137071 in a massive binary system where the more massive companion underwent a supernova explosion about 32 Myr ago.Comment: 6 pages, 4 figures. To be published in Astronomy and Astrophysic

On the possible common origin of M16 and M17

It has been suggested that the well-studied giant HII regions M16 and M17 may have had a common origin, being an example of large-scale triggered star formation. While some features of the distribution of the interstellar medium in the region support this interpretation, no definitive detection of an earlier population of massive stars responsible for the triggering has been made thus far. We have carried out observations looking for red supergiants in the area covered by a giant shell seen in HI and CO centered on galactic coordinates $l \sim 14^\circ 5$, $b\sim +1^\circ$ that peaks near the same radial velocity as the bulk of the emission from both giant HII regions, which are located along the shell. Red supergiants have ages in the range expected for the parent association whose most massive members could have triggered the formation of the shell and of the giant HII regions along its rim. Out of a sample of 37 bright red stars, we identify four red supergiants that confirm the existence of massive stars in the age range between $\sim 10$ and $\sim 30$~Myr in the area. At least three of them have Gaia DR2 parallaxes consistent with them being at the same distance as M16 and M17. The evidence of past massive star formation within the area of the gaseous shell lends support to the idea that it was formed by the combined action of stellar winds and ionizing radiation of the precursors of the current red supergiants. These could be the remnants of a richer population, whose most massive members have exploded already as core-collapse supernovae. The expansion of the shell against the surrounding medium, perhaps combined with the overrun of preexisting clouds, is thus a plausible trigger of the formation of a second generation of stars currently responsible for the ionization of M16 and M17.Comment: 7 pages, 3 figures; accepted for publication in Astronomy and Astrophysic

A very massive runaway star from Cygnus OB2

Aims: We analyze the available information on the star BD+43 3654 to investigate the possibility that it may have had its origin in the massive OB association Cygnus OB2. Methods: We present new spectroscopic observations allowing a reliable spectral classification of the star, and discuss existing MSX observations of its associated bow shock and astrometric information not previously studied. Results: Our observations reveal that BD+43 3654 is a very early and luminous star of spectral type O4If, with an estimated mass of (70 +/- 15) solar masses and an age of about 1.6 Myr. The high spatial resolution of the MSX observations allows us to determine its direction of motion in the plane of the sky by means of the symmetry axis of the well-defined bow shock, which matches well the orientation expected from the proper motion. Tracing back its path across the sky we find that BD+43 3654 was located near the central, densest region of Cygnus OB2 at a time in the past similar to its estimated age. Conclusions: BD+43 3654 turns out to be one of the three most massive runaway stars known, and it most likely formed in the central region of Cygnus OB2. A runaway formation mechanism by means of dynamical ejection is consistent with our results.Comment: Accepted by Astronomy and Astrophysics (letters); 5 pages, 3 figure

The Formation and Early Evolution of Low-mass Stars and Brown Dwarfs

The discovery of large numbers of young low-mass stars and brown dwarfs over the last decade has made it possible to investigate star formation and early evolution in a previously unexplored mass regime. In this review, we begin by describing surveys for low-mass members of nearby associations, open clusters, star-forming regions and the methods used to characterize their stellar properties. We then use observations of these populations to test theories of star formation and evolution at low masses. For comparison to the formation models, we consider the initial mass function, stellar multiplicity, circumstellar disks, protostellar characteristics, and kinematic and spatial distributions at birth for low-mass stars and brown dwarfs. To test the evolutionary models, we focus on measurements of dynamical masses and empirical Hertzsprung-Russell diagrams for young brown dwarfs and planetary companions.Comment: Final published version at http://www.annualreviews.org/journal/astr

Oxygen and silicon abundances in Cygnus OB2: Chemical homogeneity in a sample of OB slow rotators

Cygnus OB2 is a rich OB association in the Galaxy which has experienced intense star formation in the last 20-25 Myr. Its stellar population shows a correlation between age and Galactic longitude. Exploring the chemical composition of its stellar content we will be able to check the degree of homogeneity of the natal molecular cloud and possible effects of self-enrichment processes. Our aim is to determine silicon and oxygen abundances for a sample of eight early-type slow rotators in Cygnus OB2 in order to check possible inhomogeneities across the whole association and whether there exists a correlation of chemical composition with Galactic longitude. We have performed a spectroscopic analysis of a sample of late O and early B stars with low rotational velocity, which have been chosen so as to cover the whole association area. We have carried out an analysis based on equivalent widths of metal lines, the wings of the H Balmer lines and FASTWIND stellar atmosphere models to determine their stellar fundamental parameters as well as the silicon and oxygen surface abundances. We derive a rather homogeneous distribution of silicon and oxygen abundances across the region, with average values of 12+log(Si/H)=7.53$\pm$0.08 dex and 12+log(O/H)=8.65$\pm$0.12 dex. We find a homogeneous chemical composition in Cygnus OB2 with no clear evidence for significant chemical self-enrichment, despite indications of strong stellar winds and possible supernovae during the history of the region. Comparison with different scenarios of chemical enrichment by stellar winds and supernovae point to star forming efficiencies not significantly above 10%. The degree of homogeneity that we find is consistent with the observed Milky Way oxygen gradient based on HII regions. We also find that the oxygen scatter within Cygnus OB2 is at least of the same order than among HII regions at similar Galactocentric distance.Comment: 15 pages, 7 figures, accepted for publication in Astronomy & Astrophysic

The extended population associated with W40

W40 is a heavily obscured bipolar HII region projected in the direction of the Aquila Rift and ionized by hot stars in a central, partly embedded cluster. The study of the cluster and its surroundings has been greatly hampered thus far by the strong extinction in the region. We use the Gaia eDR3 catalog to establish astrometric membership criteria based on the population of the W40 central cluster, reassess the distance of the region, and identify in this way new members, both inside and outside the cluster. We obtain visible spectroscopy in the red spectral region to classify both known and new members, complemented with Gaia and Spitzer photometry to assess the evolutionary status of the stellar population. We derive a high-confidence geometric distance to the W40 region of 502 pc $\pm$ 4 pc and confirm the presence of a comoving extended population of stars at the same distance, spreading over the whole projected area of the HII region and beyond. Spectral classifications are presented for 21 members of the W40 region, 10 of them belonging to the central cluster. One of the newly identified B stars in the extended population is clearly interacting with the shell surrounding the HII region, giving rise to a small arc-shaped nebula that traces a bow shock. The infrared excess properties suggest that the extended population is significantly older ($\sim 3$ Myr) than the W40 central cluster ($< 1$ Myr). The area currently occupied by the W40 HII region and its surroundings has a history of star formation extending at least several million years in the past, of which the formation of the W40 central cluster and the subsequent HII region is one of the latest episodes. The newly determined distance suggests that W40 is behind, and physically detached from, a pervasive large dust layer which is some 60 pc foreground to it as determined by previous studies.Comment: Submitted to Astronomy and Astrophysic

Accretion in Brown Dwarfs: an Infrared View

This paper presents a study of the accretion properties of 19 very low mass objects (Mstar .01-0.1 Msun) in the regions Chamaeleon I and rho-Oph For 8 objects we obtained high resolution Halpha profiles and determined mass accretion rate Macc and accretion luminosity Lacc. Pabeta is detected in emission in 7 of the 10 rho-Oph objects, but only in one in Cha I. Using objects for which we have both a determination of Lacc from Halpha and a Pabeta detection, we show that the correlation between the Pabeta luminosity and luminosity Lacc, found by Muzerolle et al. (1998) for T Tauri stars in Taurus, extends to objects with mass approx 0.03 Msun; L(Pab) can be used to measure Lacc also in the substellar regime. The results were less conclusive for Brgamma, which was detected only in 2 objects, neither of which had an Halpha estimate of Macc. Using the relation between L(Pab) and Lacc we determined the accretion rate for all the objects in our sample (including those with no Halpha spectrum), more than doubling the number of substellar objects with known Macc. When plotted as a function of the mass of the central object together with data from the literature, our results confirm the trend of lower Macc for lower Mstar, although with a large spread. Some of the spread is probably due to an age effect; our very young objects in rho-Oph have on average an accretion rate at least one order of magnitude higher than objects of similar mass in older regions. As a side product, we found that the width of Halpha measured at 10% peak intensity is not only a qualitative indicator of accretion, but can be used to obtain a quantitative estimate of Macc over a large mass range, from T Tauri stars to brown dwarfs. Finally, we found that some of our objects show evidence of mass-loss.Comment: 11 pages, 8 figures, A&A in pres

HST, VLT, and NTT imaging search for wide companions to bona-fide and candidate brown dwarfs in the Cha I dark cloud

We present results from a deep imaging search for companions around the young bona-fide and candidate brown dwarfs Cha Ha 1 to 12 in the Cha I dark cloud, performed with HST WFPC2 (R, I, Ha), VLT FORS1 (VRI), and NTT SofI (JHK). We find 16 faint companion candidates around five primaries with separations between 1.5" and 7" and magnitudes in R & I from 19 to 25 mag, i.e. up to 8 mag fainter than the primaries. While most of these companion candidates are probably unrelated background objects, there is one promising candidate, namely 1.5" SW off the M6-dwarf Cha Ha 5. This candidate is 3.8 to 4.7 mag fainter than the primary and its colors are consistent with an early- to mid-L spectral type. Assuming the same distance (140 pc) and absorption (0.47 mag in I) as towards the primary, the companion candidate has log (L(bol)/L(odot) = -3.0 +- 0.3. At the age of the primary (1 to 5 Myrs), the faint object would have a mass of 3 to 15 Jupiter masses according to Burrows et al. (1997) and Chabrier & Baraffe (2000) models. The probability for this companion candidate to be an unrelated fore- or background object is smaller than 0.7%, its colors are marginally consistent with a strongly reddened background K giant. One other companion candidate has infrared colors consistent with an early T-dwarf. In addition, we present indications for Cha Ha 2 being a close (0.2") binary with both components very close to the sub-stellar limit. Our detection limits are such that we should have detected all companions above 1 Jup with separations above 2" (320 AU) and all above 5 Jup at 0.35" (50 AU).Comment: A&A 384, 999-1011. appeared 2002, A&A 384, 999-101

Silicate features in the circumstellar envelopes of the Class~I binary driving source of HH250

We investigate the silicate feature of the two Class I components of HH250-IRS, a resolved binary system with a separation of $0''53$ driving a Herbig-Haro flow. Each component has its own circumstellar envelope, and the system is surrounded by a circumbinary disk. We have carried out low resolution spectroscopy in the 8-13$\mu$m range using VISIR at ESO's Very Large Telescope. The silicate features of both sources are clearly different. The NW component has a broad, smooth absorption profile lacking structure. The SE component shows the silicate feature in emission, with structure longwards of 9.5$\mu$m indicating the presence of crystalline dust in the dominant form of forsterite. The apparent lack of an absorption feature caused by foreground dust is probably due to the filling of the band with emission by amorphous silicates in the envelope of the object. Despite their virtually certain coevality, the differences in the components of the HH250-IRS binary are most likely due to markedly different circumstellar environments. The NW component displays an unevolved envelope, whereas dust growth and crystallization has taken place in the SE component. The weak or absent signatures of enstatite in the latter are fairly unusual among envelopes with crystalline dust, and we tentatively relate it to a possible wide gap or an inner truncation of the disk already hinted in previous observations by a drop in the $L'$-band flux, which might indicate that the SE component could actually be a very close binary. We speculate that the clear differences between the silicate feature spectra of both components of HH250-IRS may be due either to disk evolution sped up by multiplicity, or by accretion variability leading to episodes of crystal formation.Comment: Accepted by Astronomy and Astrophysic