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Register a new userWIYN Open Cluster Study LXXXI. Caught in the Act? The Peculiar Red Giant NGC 2243-W2135
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Added by
Barbara J. Anthony-Twarog
Publication date
2020
fields
Physics
and research's language is
English
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High-dispersion spectra for giants through turnoff stars in the Li 6708 Angstrom region have been obtained and analyzed in the old, metal-deficient open cluster, NGC 2243. When combined with high dispersion data from other surveys, the cluster is found to contain a uniquely peculiar star at the luminosity level of the red clump. The giant is the reddest star at its luminosity, exhibits variability at a minimum 0.1 mag level on a timescale of days, is a single-lined, radial-velocity variable, and has V_sin(i) between 35 and 40 km/sec. In sharp contrast with the majority of the red giant cluster members, the star has a detectable Li abundance, potentially as high or higher than other giants observed to date while at or just below the boundary normally adopted for Li-rich giants. The observed anomalies may be indicators of the underlying process by which the giant has achieved its unusual Li abundance, with a recent mass transfer episode being the most probable within the currently limited constraints.
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High-dispersion spectra in the Li 6708 Angstrom region have been obtained and analyzed in the old, metal-deficient cluster, NGC 2243. From Hydra spectra for 29 astrometric and radial-velocity members, we derive rotational velocities, as well as [Fe/H], [Ca/H], [Si/H], and [Ni/H] based on 17, 1, 1, and 3 lines, respectively. Using ROBOSPECT, an automatic equivalent width measurement program, we derive [Fe/H] = -0.54 +/- 0.11 (MAD), for an internal precision for the cluster [Fe/H] below 0.03 dex. Given the more restricted line set, comparable values for [Ca/H], [Si/H], and [Ni/H] are -0.48 +/- 0.19, -0.44 +/- 0.11, and -0.61 +/- 0.06, respectively. With E(B-V) = 0.055, appropriate isochrones imply (m-M) = 13.2 +/- 0.1 and an age of 3.6 +/- 0.2 Gyr. Using available VLT spectra and published Li abundances, we construct a Li sample of over 100 stars extending from the tip of the giant branch to 0.5 mag below the Li-dip. The Li-dip is well populated and, when combined with results for NGC 6819 and Hyades/Praesepe, implies a mass/metallicity slope of 0.4 solar-mass/dex for the high mass edge of the Li-dip. The A(Li) distribution among giants reflects the degree of Li variation among the turnoff stars above the Li-dip, itself a function of stellar mass and metallicity and strongly anticorrelated with a v_rot distribution that dramatically narrows with age. Potential implications of these patterns for the interpretation of Li among dwarf and giant field populations, especially selection biases tied to age and metallicity, are discussed.
Lithium is a fundamental element for studying the mixing mechanisms acting in the stellar interiors, for understanding the chemical evolution of the Galaxy and the Big Bang nucleosynthesis. The study of Li in stars of open clusters (hereafter OC) allows a detailed comparison with stellar evolutionary models and permits us to trace its galactic evolution. The OC NGC 2243 is particularly interesting because of its low metallicity ([Fe/H]=$-0.54 pm0.10$ dex). We measure the iron and lithium abundance in stars of the metal-poor OC NGC 2243. The first aim is to determine whether the Li dip extends to such low metallicities, the second is to compare the results of our Li analysis in this OC with those present in 47 Tuc, a globular cluster of similar metallicity. We performed a detailed analysis of high-resolution spectra obtained with the multi-object facility FLAMES at the ESO VLT 8.2m telescope. Lithium abundance was derived through line equivalent widths and the OSMARCS atmosphere models. We determine a Li dip center of 1.06 $M_odot$, which is much smaller than that observed in solar metallicity and metal-rich clusters. This finding confirms and strengthens the conclusion that the mass of the stars in the Li dip strongly depends on stellar metallicity. The mean Li abundance of the cluster is $log n{rm (Li)}=2.70$ dex, which is substantially higher than that observed in 47 Tuc. We estimated an iron abundance of [Fe/H]=$-0.54 pm0.10$ dex for NGC 2243, which is similar (within the errors) to previous findings. The [$ alpha$/Fe] content ranges from $0.00pm0.14$ for Ca to $0.20pm0.22$ for Ti, which is low when compared to thick disk stars and to Pop II stars, but compatible with thin disk objects. We found a mean radial velocity of 61.9 $pm$ 0.8 kms for the cluster.
Precision uvbyCa Hbeta photometry of the metal-deficient, old open cluster, NGC 2506, is presented. The survey covers an area 20 by 20 arcminutes, and extends to V~18 for b-y and Hbeta and to V~17.0 for c_1 and hk. For V < 16.0, photometric scatter among the indices leads to the recovery of 6 known variables within the cluster core and 5 new variables in the outer 5 arcmin of the survey field. Proper motions, radial velocities, and precise multicolor indices are used to isolate a highly probable sample of cluster members from the very rich color-magnitude diagram (CMD). From 257 highly probable members at the cluster turnoff, we derive a reddening estimate of E(b-y) = 0.042 +/- 0.001 (E(B-V) = 0.058 +/- 0.001), where the errors refer to the internal standard errors of the mean. [Fe/H] is derived from the A/F dwarf members using both m_1 and hk, leading to [Fe/H] = -0.296 +/- 0.011 (sem) and -0.317 +/- 0.004 (sem), respectively. The weighted average, heavily dominated by hk, is [Fe/H] = -0.316 +/- 0.033. Based upon red giant members, we place an upper limit of +/- 0.010 on the variation in the reddening across the face of the cluster. We also identify two dozen potential red giant cluster members outside the cluster core. Victoria-Regina isochrones on the Stromgren system produce an excellent match to the cluster for an apparent modulus of (m-M) = 12.75 +/- 0.1 and an age of 1.85 +/- 0.05 Gyr.
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 to less than 20 km/s 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.
In an optical color-magnitude diagram sub-subgiants (SSGs) lie red of the main sequence and fainter than the base of the red giant branch in a region not easily populated by standard stellar-evolution pathways. In this paper, we present multi-epoch radial velocities for five SSG candidates in the old and metal-rich open cluster NGC 6791 (8 Gyr, [Fe/H] = +0.30). From these data we are able to make three-dimensional kinematic membership determinations and confirm four SSG candidates to be likely cluster members. We also identify three member SSGs as short-period binary systems and present their orbital solutions. These are the first SSGs with known three-dimensional kinematic membership, binary status, and orbital parameters since the two SSGs in M67 studied by Mathieu et al. 2003. We also remark on the other properties of these stars including photometric variability, H$alpha$ emission, and X-ray luminosity. The membership confirmation of these SSGs in NGC 6791 strengthens the case that SSGs are a new class of nonstandard stellar evolution products, and that a physical mechanism must be found that explains the evolutionary paths of these stars.
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