ABSTRACT Aiming at providing a firm mean distance estimate to the SMC, and thus to place it within the internally consistent Local Group distance framework we recently established, we compiled the ...current largest database of published distance estimates to the galaxy. Based on careful statistical analysis, we derive mean distance estimates to the SMC using eclipsing binary systems, variable stars, stellar population tracers, and star cluster properties. Their weighted mean leads to a final recommendation for the mean SMC distance of mag, where the uncertainty represents the formal error. Systematic effects related to lingering uncertainties in extinction corrections, our physical understanding of the stellar tracers used, and the SMC's complex geometry-including its significant line of sight depth, its irregular appearance which renders definition of the galaxy's center uncertain, as well as its high inclination and possibly warped disk-may contribute additional uncertainties possibly exceeding 0.15-0.20 mag.
Stars spend most of their lifetimes on the main sequence in the Hertzsprung-Russell diagram. The extended main-sequence turn-off regions--containing stars leaving the main sequence after having spent ...all of the hydrogen in their cores--found in massive (more than a few tens of thousands of solar masses), intermediate-age (about one to three billion years old) star clusters are usually interpreted as evidence of internal age spreads of more than 300 million years, although young clusters are thought to quickly lose any remaining star-forming fuel following a period of rapid gas expulsion on timescales of order 10(7) years. Here we report, on the basis of a combination of high-resolution imaging observations and theoretical modelling, that the stars beyond the main sequence in the two-billion-year-old cluster NGC 1651, characterized by a mass of about 1.7 × 10(5) solar masses, can be explained only by a single-age stellar population, even though the cluster has a clearly extended main-sequence turn-off region. The most plausible explanation for the existence of such extended regions invokes a population of rapidly rotating stars, although the secondary effects of the prolonged stellar lifetimes associated with such a stellar population mixture are as yet poorly understood. From preliminary analysis of previously obtained data, we find that similar morphologies are apparent in the Hertzsprung-Russell diagrams of at least five additional intermediate-age star clusters, suggesting that an extended main-sequence turn-off region does not necessarily imply the presence of a significant internal age dispersion.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The origin of extended main-sequence turnoffs (eMSTOs) in intermediate-age (1-3 Gyr) clusters is one of the most intriguing questions in current star cluster research. Unlike the split main sequences ...found in some globular clusters, which are caused by bimodal populations in age and/or chemical abundances, eMSTOs are believed to be due to stellar rotation. We present a spectroscopic survey of MSTO stars in a nearby, intermediate-age (0.9 Gyr), low-mass (∼1.7 × 103 M ) Galactic open cluster, NGC 5822. We derive a clean sample of member stars based on Gaia proper motions and parallaxes and confirm the existence of an eMSTO. Using medium-resolution (R ∼ 4000) Southern African Large Telescope spectra, we derive the rotational velocities of 24 member stars (representing 20% completeness around the eMSTO region) and find that the loci of the main-sequence stars in the eMSTO region show a clear correlation with the projected rotational velocities in the sense that fast rotators are located on the red side of the eMSTO and slow rotators are found on the blue side. By comparison with a synthetic cluster model, we show that the stellar rotational velocities and the eMSTO of NGC 5822 can be well reproduced, and we conclude that stellar rotation is the main cause of the eMSTO in NGC 5822.
Using high-resolution data sets obtained with the Hubble Space Telescope, we investigate the radial distributions of the F-type main-sequence binary fractions in the massive young Large Magellanic ...Cloud star clusters NGC 1805 and NGC 1818. We apply both an isochrone-fitting approach and χ2 minimization using Monte Carlo simulations, for different mass-ratio cut-offs, q, and present a detailed comparison of the methods' performance. Both methods yield the same radial binary fraction profile for the same cluster, which therefore supports the robustness and applicability of either method to young star clusters which are as yet unaffected by the presence of multiple stellar populations. The binary fractions in these two clusters are characterized by opposite trends in their radial profiles. NGC 1805 exhibits a decreasing trend with increasing radius in the central region, followed by a slow increase to the field's binary-fraction level, while NGC 1818 shows a monotonically increasing trend. This may indicate dominance of a more complicated physical mechanism in the cluster's central region than expected a priori. Time-scale arguments imply that early dynamical mass segregation should be very efficient and, hence, likely dominates the dynamical processes in the core of NGC 1805. Meanwhile, in NGC 1818 the behaviour in the core is probably dominated by disruption of soft binary systems. We speculate that this may be owing to the higher velocity dispersion in the NGC 1818 core, which creates an environment in which the efficiency of binary disruption is high compared with that in the NGC 1805 core.
Gamma-Ray Burst Progenitors Levan, Andrew; Crowther, Paul; de Grijs, Richard ...
Space science reviews,
12/2016, Letnik:
202, Številka:
1-4
Journal Article
Recenzirano
Odprti dostop
We review our current understanding of the progenitors of both long and short duration gamma-ray bursts (GRBs). Constraints can be derived from multiple directions, and we use three distinct strands; ...(i) direct observations of GRBs and their host galaxies, (ii) parameters derived from modelling, both via population synthesis and direct numerical simulation and (iii) our understanding of plausible analog progenitor systems observed in the local Universe. From these joint constraints, we describe the likely routes that can drive massive stars to the creation of long GRBs, and our best estimates of the scenarios that can create compact object binaries which will ultimately form short GRBs, as well as the associated rates of both long and short GRBs. We further discuss how different the progenitors may be in the case of black hole engine or millisecond-magnetar models for the production of GRBs, and how central engines may provide a unifying theme between many classes of extremely luminous transient, from luminous and super-luminous supernovae to long and short GRBs.
ABSTRACT Aiming at deriving a statistically well-justified Galactic Center distance, R0, and reducing any occurrence of publication bias, we compiled the most comprehensive and most complete database ...of Galactic Center distances available to date, containing 273 new or revised R0 estimates published since records began in 1918 October until 2016 June. We separate our R0 compilation into direct and indirect distance measurements. The latter include a large body of estimates that rely on centroid determinations for a range of tracer populations, as well as measurements based on kinematic observations of objects at the solar circle, combined with a mass and/or rotational model of the Milky Way. Careful assessment of the Galactic Center distances resulting from orbital modeling and statistical parallax measurements in the Galactic nucleus yields our final Galactic Center distance recommendation of . The centroid-based distances are in good agreement with this recommendation. Neither the direct measurements nor the post-1990 centroid-based distance determinations suggest that publication bias may be important. The kinematics-based distance estimates are affected by significantly larger uncertainties, but they can be used to constrain the Galaxy's rotation velocity at the solar galactocentric distance, . Our results imply that the International-Astronomical-Union-recommended Galactic Center distance ( ) needs a downward adjustment, while its recommendation ( km s−1) requires a substantial upward revision.
W Ursa Majoris (W UMa)-type contact binary systems (CBs) are useful statistical distance indicators because of their large numbers. Here, we establish (orbital) period-luminosity relations (PLRs) in ...12 optical to mid-infrared bands (GBVRIJHKsW1W2W3W4) based on 183 nearby W UMa-type CBs with accurate Tycho-Gaia parallaxes. The 1 dispersion of the PLRs decreases from optical to near- and mid-infrared wavelengths. The minimum scatter, 0.16 mag, implies that W UMa-type CBs can be used to recover distances to 7% precision. Applying our newly determined PLRs to 19 open clusters containing W UMa-type CBs demonstrates that the PLR and open cluster CB distance scales are mutually consistent to within 1%. Adopting our PLRs as secondary distance indicators, we compiled a catalog of 55,603 CB candidates, of which 80% have distance estimates based on a combination of optical, near-infrared, and mid-infrared photometry. Using Fourier decomposition, 27,318 high-probability W UMa-type CBs were selected. The resulting 8% distance accuracy implies that our sample encompasses the largest number of objects with accurate distances within a local volume with a radius of 3 kpc available to date. The distribution of W UMa-type CBs in the Galaxy suggests that in different environments, the CB luminosity function may be different: larger numbers of brighter (longer-period) W UMa-type CBs are found in younger environments.
ABSTRACT
In our recent catalogue of BY Draconis (BY Dra) variables based on Zwicky Transient Facility data, we found traces of a period gap in the period–colour diagram. We combined our BY Dra data ...base with catalogues from the Kepler and K2 surveys, revealing a prominent period gap. Here, we use this combined ZTF–Kepler–K2 data set to investigate the origin of the period gap observed for BY Dra stars using chromospheric activity indices. We use low- and medium-resolution spectra from the LAMOST Data Release 7 to derive magnetic activity indices for the Ca ii H and K and Hα emission lines. We find a strong dependence of chromospheric activity on both stellar mass and rotation period. For partially convective K–M-type stars, the activity decreases steeply up to an age of ∼700–1000 Myr, subsequently evolving to the type of low-level saturation associated with spin-down stallation. In contrast, F–G-type stars with thinner convective envelopes exhibit constant activity with increasing age. We suspect that the observed steep decrease for partially convective stars is driven by core–envelope coupling. This mechanism reduces differential rotation at the core–envelope transition, hence leading to decreased magnetic activity. Moreover, we derive activity indices for previously known star clusters and find similar trends as regards their activity levels as a function of age. In particular, very low-level activity is observed around the location of the period gap. Therefore, we conclude that the period gap, defined by the non-detection of variable sources, is driven by a minimum in chromospheric activity.
ABSTRACT
We present an extensive catalogue of BY Draconis (BY Dra)-type variables and their stellar parameters. BY Dra are main-sequence FGKM-type stars. They exhibit inhomogeneous starspots and ...bright faculae in their photospheres. These features are caused by stellar magnetic fields, which are carried along with the stellar disc through rotation and which produce gradual modulations in their light curves (LCs). Our main objective is to characterize the properties of BY Dra variables over a wide range of stellar masses, temperatures, and rotation periods. A recent study categorized 84 697 BY Dra variables from Data Release 2 of the Zwicky Transient Facility based on their LCs. We have collected additional photometric data from multiple surveys and performed broad-band spectral energy distribution fits to estimate stellar parameters. We found that more than half of our sample objects are of K spectral type, covering an extensive range of stellar parameters in the low-mass regime (0.1–1.3 M⊙). Compared with previous studies, most of the sources in our catalogue are rapid rotators, and so most of them must be young stars for which a spin-down has not yet occurred. We subdivided our catalogue based on convection zone depth and found that the photospheric activity index, Sph, is lower for higher effective temperatures, i.e. for thinner convective envelopes. We observe a broad range of photospheric magnetic activity for different spectral classes owing to the presence of stellar populations of different ages. We found a higher magnetically active fraction for K- than M-type stars.
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