Light-Element Chemistry and the Double Red Giant Branch in the Galactic Globular Cluster NGC 288Hsyu, Tiffany; Johnson, Christian I.; Lee, Young-Wook; Rich, R. Michael
doi: 10.1086/677400pmid: N/A
The globular cluster NGC 288 was previously reported to exhibit two distinct red giant branches (RGBs) in the narrowband calcium (HK) and Strömgren b- and y-band passes. In order to investigate this phenomenon further, we obtained moderate resolution (R ∼ 18,000) spectra of 27 RGB stars in NGC 288 with the Hydra multifiber spectrograph on the Blanco 4 m telescope at Cerro Tololo Inter-American Observatory (CTIO). From these data we derive iron (〈[Fe/H]〉 = -1.19; σ = 0.12), oxygen (〈[O/Fe]〉 = +0.25; σ = 0.13), and sodium (〈[Na/Fe]〉 = +0.15; σ = 0.26) abundances using standard equivalent width and spectrum synthesis techniques. Combining these data with those available in the literature indicates that the two giant branches have distinctly different light-element chemistry but do not exhibit a significant spread in [Fe/H]. A new transmission tracing for the CTIO Ca filter, obtained for this project, shows that CN contamination is the primary spectral feature driving the split RGB. Interestingly, the CN leak in the current CTIO Ca filter may be used as an efficient means to search for CN-weak and CN-strong stars in systems with otherwise small Ca abundance variations.
A Search for Mass Loss from Metal-Poor Field Red Giants using Data from the WISE and AKARI SatellitesSmith, Graeme H.; Bashay, Edward A.
doi: 10.1086/677280pmid: N/A
All-sky photometry from the WISE and AKARI satellite observatories has been used to obtain mid-infrared photometry in the 4–20 μm range for a sample of almost 400 red giants with metallicities of [Fe/H] ≤ -0.6. The mid-infrared photometry has been combined with J and KS 2MASS photometry to produce a series of two-color diagrams from which stars having circumstellar dust can be identified, namely (KS - W3, KS - W4) and (W1 - W3, W1 - W4) derived from WISE data and an analogous (KS - S9W, KS - L18W) diagram based on AKARI photometry. All three diagrams exhibit sequences of giants that are candidates for having circumstellar dust. Of the 8 metal-poor giants with the greatest mid-infrared excesses all but one are known to be in advanced stages of evolution, and are typically pulsating variables. Among giants with more modest evidence of emission in the WISE W3 bandpass, i.e., stars with 0.2 < (KS - W3) < 0.5 or (W1 - W3) > 0.10, there are also a number of known variables, but the SIMBAD data base does not list all such stars as variables. Photometric monitoring of this sample of mild-infrared-excess giants could help elucidate the potential connection between dust formation and stellar pulsation.
Homogeneous Photometry VI: Variable Stars in the Leo I Dwarf Spheroidal GalaxyStetson, Peter B.; Fiorentino, Giuliana; Bono, Giuseppe; Bernard, Edouard J.; Monelli, Matteo; Iannicola, Giacinto; Gallart, Carme; Ferraro, Ivan
doi: 10.1086/677352pmid: N/A
From archival ground-based images of the Leo I dwarf spheroidal galaxy, we have identified and characterized the pulsation properties of 164 candidate RR Lyrae variables and 55 candidate anomalous and/or short-period Cepheids. We have also identified 19 candidate long-period variable stars and 13 other candidate variables whose physical nature is unclear, but due to the limitations of our observational material we are unable to estimate reliable periods for them. On the basis of its RR Lyrae star population, Leo I is confirmed to be an Oosterhoff-intermediate type galaxy, like several other dwarf spheroidals. From the RR Lyrae stars we have derived a range of possible distance moduli for Leo I : 22.06 ± 0.08 ≲ μ0 ≲ 22.25 ± 0.07 mag depending on the metallicity assumed for the old population ([Fe/H] from -1.43 to -2.15). This is in agreement with previous independent estimates. We show that in their pulsation properties, the RR Lyrae stars—representing the oldest stellar population in the galaxy—are not significantly different from those of five other nearby, isolated dwarf spheroidal galaxies. A similar result is obtained when comparing them to RR Lyrae stars in recently discovered ultra-faint dwarf galaxies. We are able to compare the period distributions and period-amplitude relations for a statistically significant sample of ab-type RR Lyrae stars in dwarf galaxies (∼1300 stars) with those in the Galactic halo field (∼14,000 stars) and globular clusters (∼1000 stars). Field RRLs show a significant change in their period distribution when moving from the inner (dG ≲ 14 kpc) to the outer (dG ≳ 14 kpc) halo regions. This suggests that the halo formed from (at least) two dissimilar progenitors or types of progenitor. Considered together, the RR Lyrae stars in classical dwarf spheroidal and ultra-faint dwarf galaxies—as observed today—do not appear to follow the well defined pulsation properties shown by those in either the inner or the outer Galactic halo, nor do they have the same properties as RR Lyraes in globular clusters. In particular, the samples of fundamental-mode RR Lyrae stars in dwarf galaxies seem to lack High Amplitudes and Short Periods (“HASP”: AV≥1.0 mag and P ≲ 0.48 d) when compared with those observed in the Galactic halo field and globular clusters. The observed properties of RR Lyrae stars do not support the idea that currently existing classical dwarf spheroidal and ultra-faint dwarf galaxies are surviving representative examples of the original building blocks of the Galactic halo.
Calibrating Ultracool Dwarfs: Optical Template Spectra, Bolometric Corrections, and χ ValuesThis publication is partially based on observations obtain ...Schmidt, Sarah J.; West, Andrew A.; Bochanski, John J.; Hawley, Suzanne L.; Kielty, Collin
doi: 10.1086/677403pmid: N/A
We present optical template spectra, bolometric corrections, and χ values for ultracool dwarfs. The templates are based on spectra from the Sloan Digital Sky Survey (SDSS) and the Astrophysical Research Consortium 3.5 m telescope. The spectral features and overall shape of the L dwarf templates are consistent with previous spectroscopic standards and the templates have a radial velocity precision of ∼10–20 km s-1. We calculate bolometric fluxes (accurate to 10–20%) for 101 late-M and L dwarfs from SDSS, 2MASS, and WISE photometry, SDSS spectra, and BT-Settl model spectra. We find that the z-band and J-band bolometric corrections for late-M and L dwarfs have a strong correlation with z–J and J–KS colors, respectively. The new χ values, which can be used to convert Hα equivalent widths to activity strength, are based on spectrophotometrically calibrated SDSS spectra and the new bolometric fluxes. While the measured χ values have typical uncertainties of ∼20%, ultracool dwarf models show the continuum surrounding Hα can vary by up to an order of magnitude with changing surface gravity. Our semiempirical χ values are one to two orders of magnitude larger than previous χ values for mid-to-late L dwarfs, indicating that the upper limits for Hα activity strength on the coolest L dwarfs have been underestimated.