Probable Members Research Articles

Off the Beaten Path: Gaia Reveals GD-1 Stars outside of the Main Stream

Abstract Tidally disrupted globular clusters are transformed into thin, dynamically cold streams of stars that are extremely valuable tracers of the large- and small-scale distribution of mass in the Galaxy. Using data from the Gaia second data release combined with Pan-STARRS photometry, we present a sample of highly probable members of the longest cold stream in the Milky Way, GD-1. The resulting map of GD-1: (1) extends the apparent length of the stream by 20°, (2) reveals plausible locations for the progenitor, (3) detects high-contrast gaps along the stream, and (4) indicates the existence of stream members perturbed off the main stream track. These discoveries are only possible because of the exquisite astrometry from Gaia, which permits a clean separation of the stream from Milky Way stars. The additional length and a proper treatment of the progenitor will aid in dynamical modeling of GD-1 for mapping the large-scale dark matter distribution. The complex morphology of the stream points to a turbulent history; detailed phase-space properties of the perturbed stream members could potentially constrain dark matter substructure in the Milky Way.

2MASS J06562998+3002455: Not a Cool White Dwarf Candidate, but a Population II Halo Star

2MASS J06562998+3002455 or PSS 309-6 is a high proper-motion star that was discovered during a survey with the 2.1 m telescope at Kitt Peak National Observatory. Here, we reevaluate the status of this interesting star using Gaia DR2. Our results strongly suggest that PSS 309-6 could be a Population II star as the value of its V component is close to -220 km/s, which is typical for halo stars in the immediate solar neighborhood. Kapteyn's star is the nearest known halo star and PSS 309-6 exhibits similar kinematic and photometric signatures. Its properties also resemble those of 2MASS J15484023-3544254, which was once thought to be the nearest cool white dwarf but was later reclassified as K-type subdwarf. Although it is virtually certain that PSS 309-6 is not a nearby white dwarf but a more distant Population II subdwarf, further spectroscopic information, including radial velocity measurements, is necessary to fully characterize this probable member of the Galactic halo.

The First Photometric Analysis of the Open Clusters Dolidze 32 and 36

We present a first study of two open clusters Dolidze 32 and Dolidze 36 in the near-infrared region JHKs with the aid of PPMXL catalog. In our study, we used a method able to separate open cluster stars from those that belong to the stellar background. Our results of calculations indicate that for both cluster Dolidze 32 and Dolidze 36 the number of probable member is 286 and 780, respectively. We have estimated the cluster center for Dolidze 32 and Dolidze 36 are α = 18h41m4s.188 , δ = -04°04′57′′.144 , α = 20h02m29s.95 , δ = 42°05′49′′.2 , respectively. The limiting radius for both clusters Dolidze 32 and Dolidze 36 is about 0.94 ± 0.03 pc and 0.81 ± 0.03 pc, respectively. The Color Magnitude Diagram allows us to estimate the reddening E(B - V) = 1.41 ± 0.03 mag. for Dolidze 32 and E(B - V) = 0.19 ± 0.04 mag. for Dolidze 36 in such a way that the distance modulus (m - M) is 11.36 ± 0.02 and 10.10 ± 0.03 for both clusters, respectively. On the other hand, the luminosity and mass functions of these two open clusters, Dolidze 32 and Dolidze 36, have been estimated, showing that the estimated masses are 437 ± 21 M⨀ and 678 ± 26 M⨀, respectively, while the mass function slopes are -2.56 ± 0.62 and -2.01 ± 0.70 for Dolidze 32 and Dolidze 36, respectively. Finally, the dynamical state of these two clusters shows that only Dolidze 36 can be considered as a dynamically relaxed cluster.

Rotation of Low-mass Stars in Upper Scorpius and ρ Ophiuchus with K2

Abstract We present an analysis of K2 light curves (LCs) for candidate members of the young Upper Sco (USco) association (∼8 Myr) and the neighboring ρ Oph embedded cluster (∼1 Myr). We establish ∼1300 stars as probable members, ∼80% of which are periodic. The phased LCs have a variety of shapes which can be attributed to physical causes ranging from stellar pulsation and stellar rotation to disk-related phenomena. We identify and discuss a number of observed behaviors. The periods are ∼0.2–30 days with a peak near 2 days and the rapid period end nearing breakup velocity. M stars in the young USco region rotate systematically faster than GK stars, a pattern also present in K2 data for the older Pleiades and Praesepe systems. At higher masses (types FGK), the well-defined period–color relationship for slowly rotating stars seen in the Pleiades and Praesepe systems is not yet present in USco. Circumstellar disks are present predominantly among the more slowly rotating M stars in USco, with few disks in the subday rotators. However, M dwarfs with disks rotate faster on average than FGK systems with disks. For four of these disked M dwarfs, we provide direct evidence for disk locking based on the K2 LC morphologies. Our preliminary analysis shows a relatively mass-independent spin-up by a factor of ∼3.5 between USco and the Pleiades, then mass-dependent spin-down between Pleiades and Praesepe.

Chemical Abundances of New Member Stars in the Tucana II Dwarf Galaxy∗

Abstract We present chemical abundance measurements for seven stars with metallicities ranging from Fe/H] = −3.3 to [Fe/H] = −2.4 in the Tucana II ultra-faint dwarf galaxy (UFD), based on high-resolution spectra obtained with the MIKE spectrograph on the 6.5 m Magellan-Clay Telescope. For three stars, we present detailed chemical abundances for the first time. Of those, two stars are newly discovered members of Tucana II and were selected as probable members from deep narrowband photometry of the Tucana II UFD taken with the SkyMapper telescope. This result demonstrates the potential for photometrically identifying members of dwarf galaxy systems based on chemical composition. One new star was selected from the membership catalog of Walker et al. The other four stars in our sample have been reanalyzed, following additional observations. Overall, six stars have chemical abundances that are characteristic of the UFD stellar population. The seventh star shows chemical abundances that are discrepant from the other Tucana II members and an atypical, higher strontium abundance than what is expected for typical UFD stars. While unlikely, its strontium abundance raises the possibility that it may be a foreground metal-poor halo star with the same systemic velocity as Tucana II. If we were to exclude this star, Tucana II would satisfy the criteria to be a surviving first galaxy. Otherwise, this star implies that Tucana II has likely experienced somewhat extended chemical evolution.

A Deep X-ray Survey of the globular cluster Omega Centauri

We identify 233 X-ray sources, of which 95 are new, in a 222 ks exposure of Omega Centauri with the Chandra X-ray Observatory's ACIS-I detector. The limiting unabsorbed flux in the core is $f_x$ (0.5$-$6.0 keV) $\simeq$ 3 $\times$ 10$^{-16}$ erg s$^{-1}$ cm$^{-2}$ ($L_X$ $\simeq$ 1 $\times$ 10$^{30}$ erg s$^{-1}$ at 5.2 kpc). We estimate that ~$60\pm 20$ of these are cluster members, of which ~30 lie within the core ($r_c$ $=$ 155 arcsec), and another ~30 between 1$-$2 core radii. We identify four new optical counterparts, for a total of 45 likely identifications. Probable cluster members include 18 cataclysmic variables (CVs) and CV candidates, one quiescent low-mass X-ray binary, four variable stars, and five stars that are either associated with w Cen's anomalous red giant branch, or are sub-subgiants. We estimate that the cluster contains $40\pm 10$ CVs with $L_X$ $>$ 10$^{31}$ erg s$^{-1}$, confirming that CVs are underabundant in w Cen relative to the field. Intrinsic absorption is required to fit X-ray spectra of six of the nine brightest CVs, suggesting magnetic CVs, or high-inclination systems. Though no radio millisecond pulsars (MSPs) are currently known in w Cen, more than 30 unidentified sources have luminosities and X-ray colours like those of MSPs found in other globular clusters; these could be responsible for the Fermi-detected gamma-ray emission from the cluster. Finally, we identify a CH star as the counterpart to the second-brightest X-ray source in the cluster and argue that it is a symbiotic star. This is the first such giant/white dwarf binary to be identified in a globular cluster.

WIYN Open Cluster Study. LXXVI. Li Evolution Among Stars of Low/Intermediate Mass: The Metal-deficient Open Cluster NGC 2506

Abstract HYDRA spectra of 287 stars in the field of NGC 2506 from the turnoff through the giant branch are analyzed. With previous data, 22 are identified as probable binaries; 90 more are classified as potential non-members. Spectroscopic analyses of ∼60 red giants and slowly rotating turnoff stars using line equivalent widths and a neural network approach lead to [Fe/H] = −0.27 ± 0.07 (s.d.) and [Fe/H] = −0.27 ± 0.06 (s.d.), respectively. Li abundances are derived for 145 probable single-star members, 44 being upper limits. Among turnoff stars outside the Li-dip, A(Li) = 3.04 ± 0.16 (s.d.), with no trend with color, luminosity, or rotation speed. Evolving from the turnoff across the subgiant branch, there is a well-delineated decline to A(Li) ∼1.25 at the giant branch base, coupled with the rotational spindown from between ∼20 and 70 km s−1 to less than 20 km s−1 for stars entering the subgiant branch and beyond. A(Li) remains effectively constant from the giant branch base to the red giant clump level. A new member above the clump redefines the path of the first-ascent red giant branch; its Li is 0.6 dex below the first-ascent red giants. With one exception, all post-He-flash stars have upper limits to A(Li), at or below the level of the brightest first-ascent red giant. The patterns are in excellent qualitative agreement with the model predictions for low/intermediate-mass stars which undergo rotation-induced mixing at the turnoff and subgiant branch, first dredge-up, and thermohaline mixing beyond the red giant bump.

Transmission patterns of Streptococcus mutans demonstrated by a combined rep-PCR and MLST approach.

Clinical typing methods of the oral pathogen Streptococcus mutans with molecular analysis can be very specific, but expensive. Repetitive extragenic palindromic PCR (rep-PCR) is a relatively inexpensive pre-screening alternative for isolate selection for additional analyses. This study evaluated the prediction accuracy of using rep-PCR to identify S. mutans multilocus sequence typing (MLST) sequence types (ST) among children and their family members. Potential S. mutans strain sources were evaluated for evidence of transmission. Ten dendrograms (rep-PCR), with 20 isolates each of the 10 most common S. mutans genotypes, were generated from different subjects. Using a cut-off of 98% similarity, 7-11 isolates of each genotype were selected for MLST analysis to determine ST match/no-match. Overall, rep-PCR was 75% effective at determining MLST ST match/no-match and 90% effective when applied to related individuals. Most genotypes were further differentiated by MLST. MLST ST diversity was greatest for one genotype (genotype 12, G12) and evidence of transmission among children and their family members was identified by rep-PCR and MLST. Younger children (6months to 4years old) shared ST with their mothers but 50% of older children (5-9years old) had ST not identified in their mother. Six ST were shared between different families and probable source members were identified. This study confirms that rep-PCR offers an affordable option to predict diverse isolates for downstream applications. Using a combined rep-PCR and MLST approach, it is possible to track probable transmission and strain sources for S. mutans genotypes.

More Rapidly Rotating PMS M Dwarfs with Light Curves Suggestive of Orbiting Clouds of Material

Abstract In a previous paper, using data from K2 Campaign 2, we identified 11 very low mass members of the ρ Oph and Upper Scorpius star-forming region as having periodic photometric variability and phased light curves showing multiple scallops or undulations. All of the stars with the “scallop-shell” light curve morphology are mid-to-late M dwarfs without evidence of active accretion and with photometric periods generally <1 day. Their phased light curves have too much structure to be attributed to non-axisymmetrically distributed photospheric spots and rotational modulation. We have now identified an additional eight probable members of the same star-forming region plus three stars in the Taurus star-forming region with this same light curve morphology and sharing the same period and spectral type range as the previous group. We describe the light curves of these new stars in detail and present their general physical characteristics. We also examine the properties of the overall set of stars in order to identify common features that might help elucidate the causes of their photometric variability.

Star formation history of Canis Major OB1

Aims. The Canis Major OB1 Association has an intriguing scenario of star formation, especially in the region called Canis Major R1 (CMa R1) traditionally assigned to a reflection nebula, but in reality an ionized region. This work is focussed on the young stellar population associated with CMa R1, for which our previous results from ROSAT, optical, and near-infrared data had revealed two stellar groups with different ages, suggesting a possible mixing of populations originated from distinct star formation episodes. Methods. The X-ray data allow the detected sources to be characterized according to hardness ratios, light curves, and spectra. Estimates of mass and age were obtained from the 2MASS catalogue and used to define a complete subsample of stellar counterparts for statistical purposes. Results. A catalogue of 387 XMM-Newton sources is provided, of which 78% are confirmed as members or probable members of the CMa R1 association. Flares (or similar events) were observed for 13 sources and the spectra of 21 bright sources could be fitted by a thermal plasma model. Mean values of fits parameters were used to estimate X-ray luminosities. We found a minimum value of log(LX [erg/s] ) = 29.43, indicating that our sample of low-mass stars (M⋆ ≤ 0.5 M⊙), which are faint X-ray emitters, is incomplete. Among the 250 objects selected as our complete subsample (defining our “best sample”), 171 are found to the east of the cloud, near Z CMa and dense molecular gas, of which 50% of them are young (<5 Myr) and 30% are older (>10 Myr). The opposite happens to the west, near GU CMa, in areas lacking molecular gas: among 79 objects, 30% are young and 50% are older. These findings confirm that a first episode of distributed star formation occurred in the whole studied region ~10 Myr ago and dispersed the molecular gas, while a second, localized episode (<5 Myr) took place in the regions where molecular gas is still present.

Bayesian assessment of moving group membership: importance of models and prior knowledge

Young nearby moving groups are important and useful in many fields of astronomy such as studying exoplanets, low-mass stars, and the stellar evolution of the early planetary systems over tens of millions of years, which has led to intensive searches for their members. Identification of members depends on the used models sensitively, therefore, careful examination of the models is required. In this study, we investigate the effects of the models used in moving group membership calculations based on a Bayesian framework (e.g., BANYAN II) focusing on the beta-Pictoris moving group (BPMG). Three improvements for building models are suggested: (1) updating a list of accepted members by re-assessing memberships in terms of position, motion, and age, (2) investigating member distribution functions in $XYZ$, and (3) exploring field star distribution functions in $XYZ$ and $UVW$. The effect of each change is investigated, and we suggest using all of these improvements simultaneously in future membership probability calculations. Using this improved MG membership calculation and the careful examination of the age, 57 bona fide members of BPMG are confirmed including 12 new members. We additionally suggest 17 highly probable members.

Identification of Stellar Sequences in Various Stellar Systems: ESO65-SC03, Teutsch 106, Turner 6

The spatial morphological study of studied clusters is carried out through the identified probable members within them. The field stars decontamination is performed by the statistical cleaning approach (depends on the magnitude and colour of stars within the field and cluster regions). The CMRD (colour magnitude ratio diagram) approach is used to separate stellar sequences of the cluster systems. The age, distance and reddening of each cluster are estimated through the visual inspection of best fitted isochrone in colour magnitude diagrams(CMDs). The mean proper motion values of clusters are obtained through the extracted data from PPMXL and UCAC4 catalogs. Moreover, these values are varying according to the extracted data-set from these catalogues. This variation is occurred due to their different estimation efficiency of proper motions. The TCR (two colour ratio) and TCMR (two colour magnitude ratio) values of each cluster are determined by utilizing the WISE and PPMXL catalogues, these values are found abnormal for TEUTSCH 106. In addition, the TCMR values are similar to TCR values at longer wavelength, whereas both values are far away to each other at shorter wavelength. The fraction of young stellar objects (YSOs) is also computed for each cluster.

The Structure of the Young Star Cluster NGC 6231. II. Structure, Formation, and Fate

Abstract The young cluster NGC 6231 (stellar ages ∼2–7 Myr) is observed shortly after star formation activity has ceased. Using the catalog of 2148 probable cluster members obtained from Chandra, VVV, and optical surveys (Paper I), we examine the cluster’s spatial structure and dynamical state. The spatial distribution of stars is remarkably well fit by an isothermal sphere with moderate elongation, while other commonly used models like Plummer spheres, multivariate normal distributions, or power-law models are poor fits. The cluster has a core radius of 1.2 ± 0.1 pc and a central density of ∼200 stars pc−3. The distribution of stars is mildly mass segregated. However, there is no radial stratification of the stars by age. Although most of the stars belong to a single cluster, a small subcluster of stars is found superimposed on the main cluster, and there are clumpy non-isotropic distributions of stars outside ∼4 core radii. When the size, mass, and age of NGC 6231 are compared to other young star clusters and subclusters in nearby active star-forming regions, it lies at the high-mass end of the distribution but along the same trend line. This could result from similar formation processes, possibly hierarchical cluster assembly. We argue that NGC 6231 has expanded from its initial size but that it remains gravitationally bound.

Open star clusters in the Milky Way

The global survey of star clusters in the Milky Way (MWSC) is a comprehensive list of 3061 objects that provides, among other parameters, distances to clusters based on isochrone fitting. The Tycho-Gaia Astrometric Solution (TGAS) catalogue, which is a part of Gaia data release 1 (Gaia DR1), delivers accurate trigonometric parallax measurements for more than 2 million stars, including those in star clusters. We compare the open cluster photometric distance scale with the measurements given by the trigonometric parallaxes from TGAS to evaluate the consistency between these values. The average parallaxes of probable cluster members available in TGAS provide the trigonometric distance scale of open clusters, while the photometric scale is given by the distances published in the MWSC. Sixty-four clusters are suited for comparison as they have more than 16 probable members with parallax measurements in TGAS. We computed the average parallaxes of the probable members and compared these to the photometric parallaxes derived within the MWSC. We find a good agreement between the trigonometric TGAS-based and the photometric MWSC-based distance scales of open clusters, which for distances less than 2.3 kpc coincide at a level of about 0.1 mas with no dependence on the distance. If at all, there is a slight systematic offset along the Galactic equator between $30^\circ$ and $160^\circ$ galactic longitude.

A Survey For Planetary-mass Brown Dwarfs in the Taurus and Perseus Star-forming Regions*

Abstract We present the initial results from a survey for planetary-mass brown dwarfs in the Taurus star-forming region. We have identified brown dwarf candidates in Taurus using proper motions and photometry from several ground- and space-based facilities. Through spectroscopy of some of the more promising candidates, we have found 18 new members of Taurus. They have spectral types ranging from mid-M to early-L, and they include the four faintest known members in extinction-corrected K s , which should have masses as low as ∼4–5 M Jup according to evolutionary models. Two of the coolest new members (M9.25, M9.5) have mid-IR excesses that indicate the presence of disks. Two fainter objects with types of M9–L2 and M9–L3 also have red mid-IR colors relative to photospheres at ≤L0, but since the photospheric colors are poorly defined at >L0, it is unclear whether they have excesses from disks. We also have obtained spectra of candidate members of the IC 348 and NGC 1333 clusters in Perseus that were identified by Luhman et al. Eight candidates are found to be probable members, three of which are among the faintest and least-massive known members of the clusters (∼5 M Jup).

PSYM-WIDE: A Survey for Large-separation Planetary-mass Companions to Late Spectral Type Members of Young Moving Groups

Abstract We present the results of a direct imaging survey for very large separation (>100 au), low-mass companions around 95 nearby young K5–L5 stars and brown dwarfs. They are high-likelihood candidates or confirmed members of the young (≲150 Myr) β Pictoris and AB Doradus moving groups (ABDMG) and the TW Hya, Tucana–Horologium, Columba, Carina, and Argus associations. Images in and filters were obtained with the Gemini Multi-Object Spectrograph (GMOS) on Gemini South to search for companions down to an apparent magnitude of ∼ 22–24 at separations ≳20″ from the targets and in the remainder of the wide 5.′5 × 5.′5 GMOS field of view. This allowed us to probe the most distant region where planetary-mass companions could be gravitationally bound to the targets. This region was left largely unstudied by past high-contrast imaging surveys, which probed much closer-in separations. This survey led to the discovery of a planetary-mass (9–13 ) companion at 2000 au from the M3V star GU Psc, a highly probable member of ABDMG. No other substellar companions were identified. These results allowed us to constrain the frequency of distant planetary-mass companions (5–13 ) to % (95% confidence) at semimajor axes between 500 and 5000 au around young K5–L5 stars and brown dwarfs. This is consistent with other studies suggesting that gravitationally bound planetary-mass companions at wide separations from low-mass stars are relatively rare.

The Structure of the Young Star Cluster NGC 6231. I. Stellar Population

Abstract NGC 6231 is a young cluster (age ∼2–7 Myr) dominating the Sco OB1 association (distance ∼1.59 kpc) with ∼100 O and B stars and a large pre-main-sequence stellar population. We combine a reanalysis of archival Chandra X-ray data with multiepoch near-infrared (NIR) photometry from the VISTA Variables in the Vía Lactéa (VVV) survey and published optical catalogs to obtain a catalog of 2148 probable cluster members. This catalog is 70% larger than previous censuses of probable cluster members in NGC 6231. It includes many low-mass stars detected in the NIR but not in the optical and some B stars without previously noted X-ray counterparts. In addition, we identify 295 NIR variables, about half of which are expected to be pre-main-sequence stars. With the more complete sample, we estimate a total population in the Chandra field of 5700–7500 cluster members down to 0.08 (assuming a universal initial mass function) with a completeness limit at 0.5 . A decrease in stellar X-ray luminosities is noted relative to other younger clusters. However, within the cluster, there is little variation in the distribution of X-ray luminosities for ages less than 5 Myr. The X-ray spectral hardness for B stars may be useful for distinguishing between early-B stars with X-rays generated in stellar winds and B-star systems with X-rays from a pre-main-sequence companion (>35% of B stars). A small fraction of catalog members have unusually high X-ray median energies or reddened NIR colors, which might be explained by absorption from thick or edge-on disks or being background field stars.

Open cluster study in LAMOST DR3

AbstractIn studying Galactic open clusters based on LAMOST DR3, we deliberately selected several nearby cluster, which have relatively large projection area and reliable proper motion measurements. For each cluster, we firstly determine the typical proper motion distribution profiles in the cluster-core and the outskirt region, respectively, and perform field-star decontamination on the cluster area. We then calculate kinematic membership probability for each star in the cluster area and cross-match the highly probable members with LAMOST DR3 spectral catalog. Based on enhanced signal of cluster-member radial velocity distribution emerging from the whole field, we have also obtained reliable radial velocity membership probability for each star. Finally, we perform isochrones fitting with MCMC technique to study basic properties of these cluster, including age, metallicity, and distance modulus.

Proper motions of stars in the globular clusters using WFI@2.2 m telescope

AbstractWe present results of our studies for a sample of Galactic globular star clusters with the aim of deriving relative proper motions. We used CCD archival data observed with Wide Field Imager (WFI) mounted on ESO 2.2 m telescope at La Silla, Chile. Astrometric software designed by Anderson et al. is used to derive relative proper motions. The vector point diagrams show clear separation of field stars from the cluster stars. We used proper motions to determine membership probabilities and to produce color-magnitude diagrams with most probable cluster member stars. Our membership catalogue can be used to study the membership status of the peculiar stars including various variables reported in the literature.

Candidate star clusters toward the inner Milky Way discovered on deep-stackedKS-band images from the VVV Survey

The census of star clusters in the inner Milky Way is incomplete because of extinction and crowding. We embarked on a program to expand the star cluster list in the direction of the inner Milky Way using deep stacks of Ks-band images from the VISTA Variables in Via Lactea (VVV) Survey. We applied an automated two-step procedure to the point-source catalog derived from the deep Ks images: first, we identified overdensities of stars, and then we selected only candidate clusters with probable member stars that match an isochrone with a certain age, distance, and extinction on the color-magnitude diagram. This pilot project only investigates the cluster population in part of one VVV tile, that is, b201. We identified nine cluster candidates and estimated their parameters. The new candidates are compact with a typical radius on the sky of ~0.2--0.4 arcmin (~0.4-1.6 pc at their estimated distances). They are located at distances of ~5-14 kpc from the Sun and are subject to moderate extinction of E(B-V)=0.4-1.0 mag. They are sparse, probably evolved, with typical ages log(t/1 yr)~9. Based on the locations of the objects inside the Milky Way, we conclude that one of these objects is probably associated with the disk or halo and the remaining objects are associated with the bulge or the halo. The cluster candidates reported here push the VVV Survey cluster detection to the limit. These new objects demonstrate that the VVV survey has the potential to identify thousands of additional cluster candidates. The sub-arcsec angular resolution nd the near-infrared wavelength regimen give it a critical advantage over other surveys.