The elusive tidal tails of the Milky Way globular cluster NGC 7099

We present results on the extra-tidal features of the Milky Way globular cluster NGC 7099, using deep gr photometry obtained with the Dark Energy Camera (DECam). We reached nearly 6 mag below the cluster’s main sequence (MS) turnoff, so that we dealt with the most suitable candidates to trace any stellar structure located beyond the cluster tidal radius. From star-by-star reddening corrected color-magnitude diagrams (CMDs), we defined four adjacent strips along the MS, for which we built the respective stellar density maps, once the contamination by field stars was properly removed. The resulting, cleaned, field star stellar density maps show a short tidal tail and some scattered debris. Such extra-tidal features are hardly detected when much shallower Gaia DR2 data sets are used and the same CMD field star cleaning procedure is applied. Indeed, by using 2.5 mag below the MS turnoff of the cluster as the faintest limit (G < 20.5 mag), cluster members turned out to be distributed within the cluster’s tidal radius, and some hints for field star density variations are found across a circle of radius 3.5° centered on the cluster and with similar CMD features as cluster stars. The proper motion distribution of these stars is distinguishable from that of the cluster, with some superposition, which resembles that of stars located beyond 3.5° from the cluster center.

Very wide companion fraction from Gaia DR2: A weak or no enhancement for hot Jupiter hosts, and a strong enhancement for contact binaries

ABSTRACT There is an ongoing debate on whether hot Jupiter hosts are more likely to be found in wide binaries with separations of ≳100 AU. In this paper, we search for comoving, very wide companions with separations of 103–104 AU for hot Jupiter hosts and main-sequence contact binaries in Gaia DR2, and compare the very wide companion fractions with their object-by-object-matched field star samples. We find that 11.9 ± 2.5 per cent of hot Jupiter hosts and 14.1 ± 1.0 per cent of contact binaries have companions at separations of 103–104 AU. While the very wide companion fraction of hot Jupiter hosts is a factor of 1.9 ± 0.5 larger than their matched field star sample, it is consistent, within ∼1σ, with that of matched field stars if the matching is only with field stars without close companions (within ∼50 AU) as is the case for hot Jupiter hosts. The very wide companion fraction of contact binaries is a factor of 3.1 ± 0.5 larger than their matched field star sample, suggesting that the formation and evolution of contact binaries are either tied to or correlated with the presence of wide companions. In contrast, the weak enhancement of very wide companion fraction for hot Jupiter hosts implies that the formation of hot Jupiters is not as sensitive to those environment properties. Our results also hint that the occurrence rates of dual hot Jupiter hosts and dual contact binaries may be higher than the expected values from random pairing of field stars, which may be due to their underlying metallicity and age dependence.

The [α/Fe]–[Fe/H] relation in the E-MOSAICS simulations: its connection to the birth place of globular clusters and the fraction of globular cluster field stars in the bulge

ABSTRACT The α-element abundances of the globular cluster (GC) and field star populations of galaxies encode information about the formation of each of these components. We use the E-MOSAICS cosmological simulations of ∼L* galaxies and their GCs to investigate the [α/Fe]–[Fe/H] distribution of field stars and GCs in 25 Milky Way–mass galaxies. The [α/Fe]–[Fe/H] distribution of GCs largely follows that of the field stars and can also therefore be used as tracers of the [α/Fe]–[Fe/H] evolution of the galaxy. Due to the difference in their star formation histories, GCs associated with stellar streams (i.e. which have recently been accreted) have systematically lower [α/Fe] at fixed [Fe/H]. Therefore, if a GC is observed to have low [α/Fe] for its [Fe/H] there is an increased possibility that this GC was accreted recently alongside a dwarf galaxy. There is a wide range of shapes for the field star [α/Fe]–[Fe/H] distribution, with a notable subset of galaxies exhibiting bimodal distributions, in which the high [α/Fe] sequence is mostly comprised of stars in the bulge, a high fraction of which are from disrupted GCs. We calculate the contribution of disrupted GCs to the bulge component of the 25 simulated galaxies and find values between 0.3 and 14 per cent, where this fraction correlates with the galaxy’s formation time. The upper range of these fractions is compatible with observationally inferred measurements for the Milky Way, suggesting that in this respect the Milky Way is not typical of L*galaxies, having experienced a phase of unusually rapid growth at early times.

A kinematically unbiased, all-sky search for nearby, young, low-mass stars

ABSTRACT The past two decades have seen dramatic progress in our knowledge of the population of young stars of age $\lt \!200\,$ Myr that lie within $150\,$ pc of the Sun. These nearby, young stars, most of which are found in loose, comoving groups, provide the opportunity to explore (among many other things) the dissolution of stellar clusters and their diffusion into the field star population. Here, we exploit the combination of astrometric and photometric data from Gaia and photometric data from GALEX (UV) and 2MASS (near-IR) in an attempt to identify additional nearby, young, late-type stars. Specifically, we present a sample of 146 GALEX UV-selected late-type (predominantly K-type) field stars with Gaia-based distances $\lt \!125\,$ pc (based on Gaia Data Release 1) that have isochronal ages $\lt \!80\,$ Myr even if equal-components binaries. We investigate the spectroscopic and kinematic properties of this sample. Despite their young isochronal ages, only ∼10 per cent of stars among this sample can be confidently associated with established nearby, young moving groups (MGs). These candidate MG members include five stars newly identified in this study. The vast majority of our sample of 146 nearby young star candidates have anomalous kinematics relative to the known MGs. These stars may hence represent a previously unrecognized population of young stars that has recently mixed into the older field star population. We discuss the implications and caveats of such a hypothesis – including the intriguing fact that, in addition to their non-young-star-like kinematics, the majority of the UV-selected, isochronally young field stars within $50\,$ pc appear surprisingly X-ray faint.

Detection of extended Red Clump in the SMC cluster Kron 3

For the first time, we report the identification of NUV bright red clump (RC) stars and the extension of RC stars over two magnitudes both in color and magnitude axis in NUV vs (NUV – optical) color magnitude diagram. We find that the extension of RC is not due to photometric uncertainties. We suggest that the extension could be an effect of field star contamination. We also suggest that if it is an intrinsic property of the cluster then age and/or metallicity spread within the cluster could be the possible reasons for extended RC.

K2 Ultracool Dwarfs Survey – V. High superflare rates on rapidly rotating late-M dwarfs

We observed strong superflares (defined as flares with energy in excess of 1033 erg) on three late-M dwarfs: 2MASS J08315742+2042213 (hereafter 2M0831+2042; M7 V), 2MASS J08371832+2050349 (hereafter 2M0837+2050; M8 V), and 2MASS J08312608+2244586 (hereafter 2M0831+2244; M9 V). 2M0831+2042 and 2M0837+2050 are members of the young (∼700 Myr) open cluster Praesepe. The strong superflare on 2M0831+2042 has an equivalent duration (ED) of 13.7 h and an estimated energy of 1.3 × 1035 erg. We observed five superflares on 2M0837+2050, on which the strongest superflare has an ED of 46.4 h and an estimated energy of 3.5 × 1035 erg. This energy is larger by 2.7 orders of magnitude than the largest flare observed on the older (7.6 Gyr) planet-hosting M8 dwarf TRAPPIST-1. Furthermore, we also observed five superflares on 2M0831+2244 which is probably a field star. The estimated energy of the strongest superflare on 2M0831+2244 is 6.1 × 1034 erg. 2M0831+2042, 2M0837+2050, and 2M0831+2244 have rotation periods of 0.556 ± 0.002, 0.193 ± 0.000, and 0.292 ± 0.001 d, respectively, which we measured by using K2 light curves. We compare the flares of younger targets with those of TRAPPIST-1 and discuss the possible impacts of such flares on planets in the habitable zone of late-M dwarfs.

Stellar population of Sco OB2 revealed by Gaia DR2 data

Context. The Sco OB2 association is the nearest OB association, extending over approximately 2000 square degrees on the sky. Only its brightest and most massive members are already known (from HIPPARCOS) across its entire size, while studies of its lower mass population refer only to small portions of its extent. Aims. In this work we exploit the capabilities of Gaia DR2 measurements to search for Sco OB2 members across its entire size and down to the lowest stellar masses. Methods. We used both Gaia astrometric (proper motions and parallaxes) and photometric measurements (integrated photometry and colors) to select association members, using minimal assumptions derived mostly from the HIPPARCOS studies. Gaia resolves small details in both the kinematics of individual Sco OB2 subgroups and their distribution with distance from the Sun. We developed methods to explore the 3D kinematics of a stellar population covering large sky areas. Results. We find nearly 11 000 pre-main-sequence (PMS) members of Sco OB2 (with less than 3% field-star contamination), plus ∼3600 main-sequence (MS) candidate members with a larger (10–30%) field-star contamination. A higher confidence subsample of ∼9200 PMS (and ∼1340 MS) members is also selected (<1% contamination for the PMS), however this group is affected by larger (∼15%) incompleteness. We separately classify stars in compact and diffuse populations. Most members belong to one of several kinematically distinct diffuse populations, whose ensemble clearly outlines the shape of the entire association. Upper Sco is the densest region of Sco OB2. It is characterized by a complex spatial and kinematical structure and has no global pattern of motion. Other dense subclusters are found in Lower Centaurus-Crux and in Upper Centaurus-Lupus; the richest example of the latter, which has been recently identified, is coincident with the group near V1062 Sco. Most of the clustered stars appear to be younger than the diffuse PMS population, suggesting star formation in small groups that rapidly disperse and are diluted, reaching space densities lower than field stars while keeping memory of their original kinematics. We also find that the open cluster IC 2602 has a similar dynamics to Sco OB2, and its PMS members are currently evaporating and forming a diffuse (size ∼10°) halo around its double-peaked core.