An official website of the United States government
Abstract We have analyzed high-dispersion spectra in the Li 6708 Å region for 167 stars within the anticenter cluster NGC 2204. From 105 probable members, abundance analysis of 45 evolved stars produces [Fe/H] = −0.40 ± 0.12, [Si/Fe] = 0.14 ± 0.12, [Ca/Fe] = 0.29 ± 0.07, and [Ni/Fe] = −0.12 ± 0.10, where quoted errors are standard deviations. WithE(B−V) = 0.07 and (m−M)0= 13.12, appropriate isochrones provide an excellent match from the main sequence through the tip of the giant branch for an age of 1.85 ± 0.05 Gyr. Li spectrum synthesis producesA(Li) below 1.4 at the base of the red giant branch to a detectable value of −0.4 at the tip. Six probable asymptotic giant branch stars and all but one red clump star have only Li upper limits. A rapidly rotating red giant is identified as a possible Li-rich giant, assuming it is a red clump star. Main-sequence turnoff stars have a well-definedA(Li) = 2.83 ± 0.03 (sem) down to the Li-dip wall at the predicted mass of 1.29M☉. Despite having the same isochronal age as the more metal-rich NGC 2506, the luminosity distribution of red giants reflects a younger morphology similar to NGC 7789, possibly indicating a deeper impact of metallicity on stellar structure andA(Li) than previously assumed. As in NGC 2506 and NGC 7789, the NGC 2204 turnoff exhibits a broad range of rotation speeds, making abundance estimation impossible for some stars. The place of the cluster within GalacticA(Li) evolution is discussed.
more »
« less
A uvbyCaHβ CCD Analysis of the Open Cluster Standard, M67, and Its Relation to NGC 752
Abstract Precision CCDuvbyCaHβphotometry is presented of the old cluster, M67, covering one square degree with typical internal precision at the 0.005–0.020 mag level toV∼ 17. The photometry is calibrated using standards over a wide range in luminosity and temperature from NGC 752 and zeroed to the standard system via published photoelectric observations. Relative to NGC 752, differential offsets in reddening and metallicity are derived using astrometric members, supplemented by radial velocity information. From single-star members, offsets in the sense (M67−NGC 752) areδE(b−y) = −0.005 ± 0.001 (sem) mag from 327 F/G dwarfs andδ[Fe/H] = 0.062 ± 0.006 (sem) dex from the combinedm1andhkindices of 249 F dwarfs, leading toE(b−y) = 0.021 ± 0.004 (sem) and [Fe/H]M67= +0.030 ± 0.016 (sem) assuming [Fe/H]Hyades= +0.12. With probable binaries eliminated usingc1, (b−y) indices, 83 members with (π/σπ) > 50 generate (m−M)0= 8.220 ± 0.005 (sem) for NGC 752 and an isochronal age of 1.45 ± 0.05 Gyr. Using the same parallax restriction for 312 stars, M67 has (m−M) = 9.77 ± 0.02 (sem), leading to an age tied solely to the luminosity of the subgiant branch of 3.70 ± 0.03 Gyr. The turnoff color spread implies ±0.1 Gyr, but the turnoff morphology defines a younger age/higher mass for the stars, consistent with recent binary analysis and broadband photometry indicating possible missing physics in the isochrones. Anomalous stars positioned blueward of the turnoff are discussed.
more »
« less
- Award ID(s):
- 1909456
- PAR ID:
- 10396945
- Publisher / Repository:
- DOI PREFIX: 10.3847
- Date Published:
- Journal Name:
- The Astronomical Journal
- Volume:
- 165
- Issue:
- 3
- ISSN:
- 0004-6256
- Format(s):
- Medium: X Size: Article No. 105
- Size(s):
- Article No. 105
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
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
-
Abstract Age is the most difficult fundamental stellar parameter to infer for isolated stars. While isochrone-based ages are in general imprecise for both main-sequence dwarfs and red giants, precise isochrone-based ages can be obtained for stars on the subgiant branch transitioning from core to shell hydrogen burning. We synthesize Gaia Data Release 3–based distance inferences, multiwavelength photometry from the ultraviolet to the mid-infrared, and three-dimensional extinction maps to construct a sample of 289,759 solar-metallicity stars amenable to accurate, precise, and physically self-consistent age inferences. Using subgiants in the solar-metallicity open clusters NGC 2682 (i.e., M67) and NGC 188, we show that our approach yields accurate and physically self-consistent ages and metallicities with median statistical precisions of 8% and 0.06 dex, respectively. The inclusion of systematic uncertainties resulting from nonsingle or variable stars results in age and metallicity precisions of 9% and 0.12 dex, respectively. We supplement this solar-metallicity sample with an additional 112,062 metal-poor subgiants, including over 3000 stars with [Fe/H] ≲ −1.50, 7% age precisions, and apparent GaiaG-band magnitudesG< 14. We further demonstrate that our inferred metallicities agree with those produced by multiplexed spectroscopic surveys. As an example of the scientific potential of this catalog, we show that the solar neighborhood star formation history has three components at ([Fe/H],τ/Gyr) ≈ (+0.0, 4), (+0.2, 7), and a roughly linear sequence in age–metallicity space beginning at ([Fe/H],τ/Gyr) ≈ (+0.2, 7) and extending to (−0.5, 13). Our analyses indicate that the solar neighborhood includes stars on disk-like orbits even at the oldest ages and lowest metallicities accessible by our samples.more » « less
-
Abstract We present stellar rotation periods for late K- and early M-dwarf members of the 4 Gyr old open cluster M67 as calibrators for gyrochronology and tests of stellar spin-down models. Using Gaia EDR3 astrometry for cluster membership and Pan-STARRS (PS1) photometry for binary identification, we build this set of rotation periods from a campaign of monitoring M67 with the Canada–France–Hawaii Telescope’s MegaPrime wide-field imager. We identify 1807 members of M67, of which 294 are candidate single members with significant rotation period detections. Moreover, we fit a polynomial to the period versus color-derived effective temperature sequence observed in our data. We find that the rotation of very cool dwarfs can be explained by simple solid-body spin-down between 2.7 and 4 Gyr. We compare this rotational sequence to the predictions of gyrochronological models and find that the best match is Skumanich-like spin-down,Prot∝t0.62, applied to the sequence of Ruprecht 147. This suggests that, for spectral types K7–M0 with near-solar metallicity, once a star resumes spinning down, a simple Skumanich-like relation is sufficient to describe their rotation evolution, at least through the age of M67. Additionally, for stars in the range M1–M3, our data show that spin-down must have resumed prior to the age of M67, in conflict with the predictions of the latest spin-down models.more » « less
-
Abstract We present near-infraredJHKphotometry for the resolved stellar populations in 13 nearby galaxies: NGC 6822, IC 1613, NGC 3109, Sextans B, Sextans A, NGC 300, NGC 55, NGC 7793, NGC 247, NGC 5253, Cen A, NGC 1313, and M83, acquired from the 6.5 m Baade–Magellan telescope. We measure distances to each galaxy using the J-region asymptotic giant branch (JAGB) method, a new standard candle that leverages the constant luminosities of color-selected, carbon-rich AGB stars. While only single-epoch, random-phase photometry is necessary to derive JAGB distances, our photometry is time-averaged over multiple epochs, thereby decreasing the contribution of the JAGB stars’ intrinsic variability to the measured dispersions in their observed luminosity functions. To cross-validate these distances, we also measure near-infrared tip of the red giant branch (TRGB) distances to these galaxies. The residuals obtained from subtracting the distance moduli from the two methods yield an rms scatter ofσJAGB−TRGB= ±0.07 mag. Therefore, all systematics in the JAGB method and TRGB method (e.g., crowding, differential reddening, star formation histories) must be contained within these ±0.07 mag bounds for this sample of galaxies because the JAGB and TRGB distance indicators are drawn from entirely distinct stellar populations and are thus affected by these systematics independently. Finally, the composite JAGB star luminosity function formed from this diverse sample of galaxies is well described by a Gaussian function with a modal value ofMJ= –6.20 ± 0.003 mag (stat), indicating that the underlying JAGB star luminosity function of a well-sampled full star formation history is highly symmetric and Gaussian based on over 6700 JAGB stars in the composite sample.more » « less
