The Macho Project LMC variable star inventory. II. LMC RR Lyrae stars{emdash}pulsational characteristics and indications of a global youth of the LMC

Journal Article · Fri Mar 01 04:00:00 EST 1996 · Astronomical Journal

DOI:https://doi.org/10.1086/117859· OSTI ID:278887

Alcock, C ^[1]; Allsman, R A ^[2]; Axelrod, T S ^[1]; Bennett, D P; Cook, K H ^[1]; Freeman, K C ^[3]; Griest, K ^[4]; Marshall, S L ^[5]; Peterson, B A ^[6]; Pratt, M R ^[5]; Quinn, P J; Rodgers, A W ^[7]; Stubbs, C W ^[8]; Sutherland, S ^[9]; Welch, D L ^[10]

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1. Lawrence Livermore National Laboratory, Livermore, California 94550 (United States)
2. Super Computer Facility, Australian National University, Canberra, ACT 0200 (Australia)
3. Mount Stromlo and Spring Observatories, Australian National University, Weston, ACT 2611 (Australia)
4. Department of Physics, University of California, San Diego, California 92093 (United States)
5. Department of Physics, University of California, Santa Barbara, California 93106 (United States)
6. Mount Stromlo and Siding Spring Observatories, Australian National University, Weston, ACT 2611 (Australia)
7. Mount Stromlo and Siding Spring Observatories, Australian National University, Weston, Act 2611 (Australia)
8. Departments of Astronomy and Physics, University of Washington, Seattle, Washington 98195 (United States)
9. Department of Physics, University of Oxford, Oxford OX13RH (United Kingdom)
10. Department of Physics and Astronomy, McMaster University, Hamilton, Ontario L8S4M1 (Canada)

The Macho Project Year One Database contains photometric observations of twenty-two 40 arcmin fields in the bar region of the LMC which are analyzed to select stars with the properties of LMC field RR Lyrae stars. The resulting set of {approximately}7900 stars, in the range of brightness 18.7{lt}{ital V}{lt}19.7, is phased and the distribution of periods determined. There is an excess of stars with periods near 0.28{sup d} which show skewed light curves predicted [Stellingwerf {ital et} {ital al}. ApJ, 313, L75 (1987)] to be characteristic of second-harmonic pulsations of RR Lyrae stars. From a sample of 500 stars examined in detail, we have found two double-mode RR Lyrae stars pulsating in both fundamental and first overtone modes. The period ratios, {ital P}{sub 1}/{ital P}{sub 0}=0.74260 and 0.74225 at {ital P}{sub 0}=0{sup d}.46118 and 0{sup d}.46318, respectively, are lower than for any Galactic beat RR Lyrae star. The period distribution of the LMC field RR Lyrae stars shows no Oosterhof dichotomy and has a most frequent period of type a,b stars at 0.583{sup d}, intermediate between the Galactic preferred periods of Oosterhof groups I ({ital P}=0.55{sup d}) and II ({ital P}=0.65{sup d}). Detailed examination of 500 randomly chosen light curves shows that the period amplitude relation is dominated by low amplitude Bailey type b stars indicative of a red morphology of the field horizontal branch of the LMC. Spectroscopic abundances of 15 field LMC RR Lyrae stars are derived to obtain a modal [Fe/H]{approximately}{minus}1.7 which together with the red HB morphology indicates that the field RR Lyrae population of the LMC is at the younger extreme of the age range of the outer clusters of the Galactic halo and suggests that the field HB population is younger, by about 1 Gyr, than the old cluster population of the LMC. {copyright} {ital 1996 American Astronomical Society.}

• Research Organization: Lawrence Livermore National Laboratory
• DOE Contract Number: W-7405-ENG-48
• OSTI ID: 278887
• Journal Information: Astronomical Journal, Journal Name: Astronomical Journal Journal Issue: 3 Vol. 111; ISSN 0004-6256; ISSN ANJOAA
• Country of Publication: United States
• Language: English

Related Subjects

66 PHYSICS
AGE ESTIMATION
ELEMENT ABUNDANCE
GALACTIC EVOLUTION
LIGHT CURVES
MAGELLANIC CLOUDS
PERIODIC VARIATIONS
PERIODS
PULSATIONS
RR LYRAE STARS
VARIABLE STARS