skip to main content

Explore Research Products in the NSF-PAR

Title: A Census of Blue Stragglers in Gaia DR2 Open Clusters as a Test of Population Synthesis and Mass Transfer Physics
Abstract We use photometry and proper motions from Gaia DR2 to determine the blue straggler star (BSS) populations of 16 old (1–10 Gyr), nearby ( d < 3500 pc) open clusters. We find that the fractional number of BSS compared to red giant branch stars increases with age, starting near zero at 1 Gyr and flattening to ∼0.35 by 4 Gyr. Fitting stellar evolutionary tracks to these BSSs, we find that their mass distribution peaks at a few tenths of a solar mass above the main-sequence turnoff. BSSs more than 0.5 M ⊙ above the turnoff make up only ∼25% of the sample, and BSSs more than 1.0 M ⊙ above the turnoff are rare. We compare this to Compact Object Synthesis and Monte Carlo Investigation Code population synthesis models of BSSs formed via mass transfer. We find that standard population synthesis assumptions dramatically under-produce the number of BSS in old open clusters. We also find that these models overproduce high-mass BSSs relative to lower-mass BSSs. The expected number of BSSs formed through dynamics do not fully account for this discrepancy. We conclude that in order to explain the observed BSS populations from Roche lobe overflow, mass transfer from giant donors must be more stable than assumed in canonical mass-transfer prescriptions, and including nonconservative mass transfer is important in producing realistic BSS masses. Even with these modifications, it is difficult to achieve the large number of BSSs observed in the oldest open clusters. We discuss some additional physics that may explain the large number of observed blue stragglers among old stellar populations.  more » « less
Award ID(s):
1801937
NSF-PAR ID:
10343809
Author(s) / Creator(s):
;
Date Published:
Journal Name:
The Astrophysical Journal
Volume:
908
Issue:
2
ISSN:
0004-637X
Page Range / eLocation ID:
229
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. ; ( , The Astrophysical Journal)
    Abstract

    WOCS 4540 is the longest orbital period (Porb= 3030 days) blue straggler star (BSS)—white dwarf (WD) pair in the old open cluster NGC 188. It also contains one of the most luminous BSS in the cluster. Prior Hubble Space Telescope Cosmic Origins Spectrograph spectroscopy measured a WD mass of 0.53M⊙, indicative of a carbon–oxygen WD and suggesting previous mass transfer from an asymptotic giant branch (AGB) star. Detailed modeling of the system evolution, including red giant branch phase wind mass transfer, AGB wind Roche-lobe overflow, and regular Roche-lobe overflow, is done with Modules for Experiments in Stellar Astrophysics. The best-fit model produces excellent agreement with a wide array of observational constraints on the BSS, the WD, and the binary system. To produce the observed luminosity and effective temperature of the BSS, all three donor mass-transfer mechanisms contribute similarly to build a 1.5M⊙BSS. The overall mass-transfer efficiency is 55%. Regular Roche-lobe overflow occurs only during the largest AGB thermal pulse, but yields a very high accretion rate at 75% efficiency and briefly (less than 1 Myr) a very high luminosity boost from the accretor.

     
    more » « less
  2. ; ; ; ; ; ; ; ; ; ; et al ( , The Astrophysical Journal)
    Abstract Recent work has shown that near-infrared (NIR) Hubble Space Telescope (HST) photometry allows us to disentangle multiple populations (MPs) among M dwarfs of globular clusters (GCs) and to investigate this phenomenon in very-low-mass (VLM) stars. Here, we present the color–magnitude diagrams of nine GCs and the open cluster NGC 6791 in the F110W and F160W bands of HST, showing that the main sequences (MSs) below the knee are either broadened or split, thus providing evidence of MPs among VLM stars. In contrast, the MS of NGC 6791 is consistent with a single population. The color distribution of M dwarfs dramatically changes between different GCs, and the color width correlates with the cluster mass. We conclude that the MP ubiquity, variety, and dependence on GC mass are properties common to VLM and more-massive stars. We combined UV, optical, and NIR observations of NGC 2808 and NGC 6121 (M4) to identify MPs along with a wide range of stellar masses (∼0.2–0.8  ⊙ ), from the MS turnoff to the VLM regime, and measured, for the first time, their mass functions (MFs). We find that the fraction of MPs does not depend on the stellar mass and that their MFs have similar slopes. These findings indicate that the properties of MPs do not depend on stellar mass. In a scenario where the second generations formed in higher-density environments than the first generations, the possibility that the MPs formed with the same initial MF would suggest that it does not depend on the environment. 
    more » « less
  3. ; ; ; ; ; ; ; ; ; ; et al ( , The Astronomical Journal)
    Abstract

    We present analyses of improved photometric and spectroscopic observations for two detached eclipsing binaries at the turnoff of the open cluster NGC 752: the 1.01 days binary DS And and the 15.53 days BD +37 410. For DS And, we findM1= 1.692 ± 0.004 ± 0.010M⊙,R1= 2.185 ± 0.004 ± 0.008R⊙,M2= 1.184 ± 0.001 ± 0.003M⊙, andR2= 1.200 ± 0.003 ± 0.005R⊙. We either confirm or newly identify unusual characteristics of both stars in the binary: the primary star is found to be slightly hotter than the main-sequence turnoff and there is a more substantial discrepancy in its luminosity compared to models (model luminosities are too large by about 40%), while the secondary star is oversized and cooler compared to other main-sequence stars in the same cluster. The evidence points to nonstandard evolution for both stars, but most plausible paths cannot explain the low luminosity of the primary star. BD +37 410 only has one eclipse per cycle, but extensive spectroscopic observations and the Transiting Exoplanet Survey Satellite light curve constrain the stellar masses well:M1= 1.717 ± 0.011M⊙andM2= 1.175 ± 0.005M⊙. The radius of the main-sequence primary star near 2.9R⊙definitively requires large convective core overshooting (>0.2 pressure scale heights) in models for its mass, and multiple lines of evidence point toward an age of 1.61 ± 0.03 ± 0.05 Gyr (statistical and systematic uncertainties). Because NGC 752 is currently undergoing the transition from nondegenerate to degenerate He ignition of its red clump stars, BD +37 410 A directly constrains the star mass where this transition occurs.

     
    more » « less
  4. ; ; ; ; ; ; ; ; ; ( , The Astrophysical Journal)
    Abstract

    We used high-resolution spectra acquired with the Magellan Telescope to measure radial and rotational velocities of approximately 200 stars in the Galactic globular cluster NGC 3201. The surveyed sample includes blue straggler stars (BSSs) and reference stars in different evolutionary stages (main-sequence turnoff, subgiant, red giant, and asymptotic giant branches). The average radial velocity value (〈Vr〉 = 494.5 ± 0.5 km s−1) confirms a large systemic velocity for this cluster and was used to distinguish 33 residual field interlopers. The final sample of member stars has 67 BSSs and 114 reference stars. Similarly to what is found in other clusters, the totality of the reference stars has negligible rotation (< 20 km s−1), while the BSS rotational velocity distribution shows a long tail extending up to ∼200 km s−1, with 19 BSSs (out of 67) spinning faster than 40 km s−1. This sets the percentage of fast-rotating BSSs to ∼28%. Such a percentage is roughly comparable to that measured in other loose systems (ωCentauri, M4, and M55) and significantly larger than that measured in high-density clusters (as 47 Tucanae, NGC 6397, NGC 6752, and M30). This evidence supports a scenario where recent BSS formation (mainly from the evolution of binary systems) is occurring in low-density environments. We also find that the BSS rotational velocity tends to decrease for decreasing luminosity and surface temperature, similarly to what is observed in main-sequence stars. Hence, further investigations are needed to understand the impact of BSS internal structure on the observed rotational velocities.

     
    more » « less
  5. ; ; ; ; ; ; ; ; ; ; et al ( , Astronomy & Astrophysics)
    Aims. The Blanco DECam Bulge Survey (BDBS) has imaged more than 200 square degrees of the southern Galactic bulge, providing photometry in the ugrizy filters for ∼250 million unique stars. The presence of a strong foreground disk population, along with complex reddening and extreme image crowding, has made it difficult to constrain the presence of young and intermediate age stars in the bulge population. Methods. We employed an accurate cross-match of BDBS with the latest data release (EDR3) from the Gaia mission, matching more than 140 million sources with BDBS photometry and Gaia EDR3 photometry and astrometry. We relied on Gaia EDR3 astrometry, without any photometric selection, to produce clean BDBS bulge colour-magnitude diagrams (CMDs). Gaia parallaxes were used to filter out bright foreground sources, and a Gaussian mixture model fit to Galactic proper motions could identify stars kinematically consistent with bulge membership. We applied this method to 127 different bulge fields of 1 deg 2 each, with | ℓ | ≤ 9.5° and −9.5° ≤ b  ≤ −2.5°. Results. The astrometric cleaning procedure removes the majority of blue stars in each field, especially near the Galactic plane, where the ratio of blue to red stars is ≲10%, increasing to values ∼20% at higher Galactic latitudes. We rule out the presence of a widespread population of stars younger than 2 Gyr. The vast majority of blue stars brighter than the turnoff belong to the foreground population, according to their measured astrometry. We introduce the distance between the observed red giant branch bump and the red clump as a simple age proxy for the dominant population in the field, and we confirm the picture of a predominantly old bulge. Further work is needed to apply the method to estimate ages to fields at higher latitudes, and to model the complex morphology of the Galactic bulge. We also produce transverse kinematic maps, recovering expected patterns related to the presence of the bar and of the X-shaped nature of the bulge. 
    more » « less
  • Citation Formats
  • MLA
  • APA
  • Chicago
  • BibTeX
  • Save / Share this Record
  • Send to Email