Christine M. Clement

VARIABLE STARS IN GALACTIC GLOBULAR CLUSTERS

Based on a search of the literature up to 2001 May, the number of known variable stars in Galactic globular clusters is approximately 3000. Of these, more than 2200 have known periods and the majority (approximately 1800) are of the RR Lyrae type. In addition to the RR Lyrae population, there are approximately 100 eclipsing binaries, 120 SX Phoenicis variables, 60 Cepheids (including Population II Cepheids, anomalous Cepheids and RV Tauri), and 120 SR/red variables. The mean period of the fundamental mode RR Lyrae variables is 0.585 days, for the overtone variables it is 0.342 days (0.349 days for the first-overtone pulsators and 0.296 days for the second-overtone pulsators) and approximately 30% are overtone pulsators. These numbers indicate that about 65% of RR Lyrae variables in Galactic globular clusters belong to Oosterhoff type I systems. The mean period of the RR Lyrae variables in the Oosterhoff type I clusters seems to be correlated with metal abundance in the sense that the periods are longer in the more metal poor clusters. Such a correlation does not exist for the Oosterhoff type II clusters. Most of the Cepheids are in clusters with blue horizontal branches.

THE MACHO PROJECT LARGE MAGELLANIC CLOUD VARIABLE-STAR INVENTORY. IX. FREQUENCY ANALYSIS OF THE FIRST-OVERTONE RR LYRAE STARS AND THE INDICATION FOR NONRADIAL PULSATIONS

More than 1300 variables classified provisionally as first-overtone RR Lyrae pulsators in the MACHO variable-star database of the Large Magellanic Cloud (LMC) have been subjected to standard frequency analysis. Based on the remnant power in the prewhitened spectra, we found 70% of the total population to be monoperiodic. The remaining 30% (411 stars) are classified as one of nine types according to their frequency spectra. Several types of RR Lyrae pulsational behavior are clearly identified here for the first time. Together with the earlier discovered double-mode (fundamental and first-overtone) variables, this study increased the number of known double-mode stars in the LMC to 181. During the total 6.5 yr time span of the data, 10% of the stars showed strong period changes. The size, and in general also the patterns of the period changes, exclude a simple evolutionary explanation. We also discovered two additional types of multifrequency pulsators with low occurrence rates of 2% for each. In the first type, there remains one closely spaced component after prewhitening by the main pulsation frequency. In the second type, the number of remnant components is two; they are also closely spaced, and are symmetric in their frequency spacing relative to the central component. This latter type of variables are associated with their relatives among the fundamental pulsators, known as Blazhko variables. Their high frequency (≈20%) among the fundamental-mode variables versus the low occurrence rate of their first-overtone counterparts makes it more difficult to explain the Blazhko phenomenon by any theory depending mainly on the role of aspect angle or magnetic field. None of the current theoretical models are able to explain the observed close frequency components without invoking nonradial pulsation components in these stars.

A provisional RR lyrae distance scale

Hydrodynamic pulsation models are matched with observations of globular cluster RRc stars to derive masses, luminosities and temperatures as functions of two observables: pulsation period, P1, and Fourier phase parameter, phi sub 31. We find that mean RRc masses and luminosities increase, and mean temperatures fall, with decreasing cluster metallicity. The Oosterhoff dichotomy is explained as mainly a temperature effect, while hints of a new dichotomy (in RRc mass and luminosity) are found among certain Oo II clusters. A provisional RR Lyrae distance scale emerges for the RRc stars in the form of a relation between luminosity and the two observables, P1 and phi sub 31. Finally, the scheme we propose passes three independent tests: (1) it reproduces the observed hierarchy of relative luminosity among a large sample of RRc stars in Omega Centauri; (2) it yields mean RRc masses which are fully consistent with the RRd masses derived for the clusters M68 and M15; and (3) it gives an LMC distance modulus in agreement with that obtained by other methods.

The Globular Cluster ω Centauri and the Oosterhoff Dichotomy

CCD observations obtained by the OGLE team for 128 RR Lyrae variables in ? Centauri have been analyzed. The period-luminosity and period-amplitude plots indicate that, in addition to fundamental (RRab) and first overtone (RRc) pulsators, the ? Centauri RR Lyrae population seems to include second overtone (RRe) and possibly third-overtone pulsators. The mean period for the 59 RRab stars is 0649, for the 48 RRc stars, it is 0383, and for the 21 RRe stars, it is 0304. The mean periods derived for the RRab and RRc stars are typical values for an Oosterhoff type II (OoII) cluster. Nevertheless, the period-amplitude plot also shows that some of the RR Lyrae variables have Oosterhoff type I (OoI) characteristics. Most of the second-overtone variables exhibit nonradial pulsations similar to those recently detected in some of the RR Lyrae variables in the clusters M55 and M5, in the Galactic bulge, and in the LMC. Relative luminosities derived for the RRc variables from Fourier coefficients correlate with the observed apparent magnitudes. Masses for the RRc stars have been calculated from Fourier coefficients. A comparison of the derived masses for RRc stars in the four OoII clusters ? Cen, M15, M55, and M68 indicates that the masses of the RRc stars in M15 and M68 are almost 0.2 M? greater than those in the other two. Since M15 and M68 have a high frequency of RRd stars among their first-overtone pulsators, while none have been identified in ? Cen or M55, this suggests that the double-mode pulsation phenomenon may be associated with mass. Among the RRc variables in ? Cen, the OoII variables have lower derived masses and higher luminosities than the OoI variables. An application of the period-density law to pairs of OoI and OoII RRab stars selected according to their position in the period-amplitude plot also indicates that the OoII variables in general have lower masses and higher luminosities. These findings support the hypothesis that the RR Lyrae variables in OoII systems are evolved horizontal-branch stars that spend their zero-age horizontal-branch phase on the blue side of the instability strip.

The MACHO Project Large Magellanic Cloud Variable-Star Inventory. XIII. Fourier Parameters for the First-Overtone RR Lyrae Variables and the LMC Distance

Shapes of RR Lyrae light curves can be described in terms of Fourier coefficients that past research has linked with physical characteristics such as luminosity, mass, and temperature. Fourier coefficients have been derived for the V and R light curves of 785 overtone RR Lyrae variables in 16 MACHO fields near the bar of the LMC. In general, the Fourier phase differences 21, 31, and 41 increase and the amplitude ratio R21 decreases with increasing period. The coefficients for both the V and R magnitudes follow these patterns, but the phase differences for the R curves are on average slightly greater, and their amplitudes are about 20% smaller, than the ones for the V curves. The 31 and R21 coefficients have been compared with those of the first-overtone RR Lyrae variables in the Galactic globular clusters NGC 6441, M107, M5, M3, M2, ω Centauri, and M68. The results indicate that many of the LMC variables have properties similar to the ones in M2, M3, M5, and the Oosterhoff type I variables in ω Cen, but they are different from the Oosterhoff type II variables in ω Cen. Equations derived from hydrodynamic pulsation models have been used to calculate the luminosity and temperature for the 330 bona fide first-overtone variables. The results indicate that they have log L in the range 1.6–1.8 L⊙ and log Teff between 3.85 and 3.87. Based on these temperatures, a mean color excess E(V-R) = 0.08 mag, equivalent to E(B-V) = 0.14 mag, has been estimated for these 330 stars. The 80 M5-like variables (selected according to their location in the 31–log P plot) are used to determine an LMC distance. After correcting for the effects of extinction and crowding, a mean apparent magnitude V0 = 18.99 ± 0.02 (statistical) ±0.16 (systematic) has been estimated for these 80 stars. Combining this with a mean absolute magnitude MV = 0.56 ± 0.06 for M5-like stars derived from Baade-Wesselink analyses, main-sequence fitting, Fourier parameters, and the trigonometric parallax of RR Lyrae, we derive an LMC distance modulus μ = 18.43 ± 0.06 (statistical) ±0.16 (systematic) mag. The large systematic error arises from the difficulties of correcting for interstellar extinction and for crowding.

Period Changes in ω Centauri RR Lyrae Stars

A century of observations of ω Centauri allows us to accurately follow the period changes of a large sample of variable stars. Although the period changes are composed of monotone and irregular changes, period increases dominate in most of the RRab stars. The typical period increase rates are of the same order of magnitude as model calculations predict for the evolved, redward phase of horizontal-branch evolution. For the 44 well-observed RRab stars that show monotonic period changes, a mean rate of 0.15 days Myr-1 has been found. The period changes of the first-overtone RRc-type stars show a much more complex, irregular behavior.

Long-term photometric monitoring of Messier 5 variables – II. Blazhko stars

The light curves of 50 RRab (RRO) stars in M5 collected in Szeidl et al. (Paper I) are investigated to detect Blazhko modulation. 18 Blazhko stars are identified, and modulation is suspected in two additional cases. The mean pulsation period of Blazhko stars is 0.04 d shorter than the mean period of the entire RRab sample in M5. Among the RRab stars with period shorter than 0.55 d the incidence rate of the modulation is as high as 60 per cent. The mean B - V colours of Blazhko stars overlap with the colours of first overtone RRc (RR1) pulsators. The mean V magnitudes of Blazhko stars are on the average 0.05-mag fainter than those of the RRab stars with stable light curves. Blazhko stars tend to be situated close to the zero-age horizontal branch at the blue edge of the fundamental-mode instability strip in M5. We speculate that this specific location hints that the Blazhko effect may have an evolutionary connection with the mode switch from the fundamental to the overtone-mode pulsation.

THE RR LYRAE VARIABLES IN THE GLOBULAR CLUSTER M68

New observations, made with the Helen Sawyer Hogg telescope at Las Campanas, have been analyzed in a search for double-mode pulsators (RRd stars) in the metal-poor globular cluster, Messier 68. Of the 30 stars studied, nine have been identified as RRd stars; V33, which exhibited the characteristics of an RRd star in 1950, now appears to be an RRc star. Reliable periods and period ratios have been determined for six of the RRd stars. Masses for these RRd stars, calculated from fitting formulas given by Kovacs et al. (1991), range from 0.75 to 0.90 solar mass, depending on the assumed luminosity and metal abundance. These masses are in the same range as those for the RRd stars in M 15, whose RRd sample resembles that of M68 very closely. Fourier parameters determined for the light curves of the M68 variables show that the RRc stars in the two clusters are also very similar. In particular, on the plot of phase parameter phi sub 31 with period, the M15 and M68 RRc samples are virtually indistinguishable. A comparison of the new M68 observations with observations made 40 yr ago shows that the periods of some of the stars have changed, but the 40 yr interval is too short for detecting period changes caused by evolutionary effects.

The Mode Change of the RR Lyrae Variable V79 in M3

The RR Lyrae variable V79 in M3 has been studied on approximately 1000 photographs in the Sternberg Observatory plate collection and on 126 CCD frames. The photographs were obtained over the interval 1949-1992, and the CCD observations were made in 1998. A comparison of our data with observations made in 1992, 1993, and 1996 indicates that V79 changed its pulsation mode in 1992. Before 1992, it was a fundamental mode pulsator (RRab star) with an irregular light curve, but since then it has been a double-mode pulsator (RRd star) with the first overtone the dominant mode. Prior to the mode change, there were sporadic episodes when pulsation in the first overtone mode occurred, for example, for a few days in 1976 and again in 1989. In each case, however, the fundamental mode pulsation resumed afterward. During the first two weeks in April of 1989, the star appeared to stop varying, and when it began again, the period of the fundamental mode pulsation had decreased substantially, from 0.483 to 0.480 days. Although the first overtone has been the dominant mode of pulsation for V79 since 1992 when it became an RRd star, our 1998 observations show that the amplitude of the fundamental mode is greater than it was in 1996. Previously published investigations of the Oosterhoff type II cluster M15 indicate that some of its RRd stars have exhibited irregular behavior, but no mode switches have been observed. In addition, there are some short period RRab stars in other Oosterhoff type I clusters that have changed their light curve amplitude and shape on a timescale of years, but up to now, none of these have been shown to be RRd stars.

Long-term photometric monitoring of Messier 5 variables - I. Period changes of RR Lyrae stars

The period changes of 86 M5 RR Lyrae stars have been investigated on a 100-yr time base. The published observations have been supplemented by archival Asiago, Konkoly and Las Campanas photographic observations obtained between 1952 and 1993. About two-thirds of the O - C diagrams could be fitted by a straight line or a parabola. 21 RR Lyrae stars have increasing, 18 decreasing and 16 constant period. The mean rates of period change of these variables are: 〈β〉 = 〈P〉 = -0.006 ± 0.162 d Myr -1 , 〈α〉 = 〈P -1 P〉 = -0.021 ± 0.308 Myr -1 . 10 RR Lyrae stars show fast period decrease with P < -0.10 d Myr -1 . At least some of these variables may be in the pre-zero-age horizontal-branch (ZAHB) evolutionary stage. The variables on the long-period sequence of the period-amplitude diagram are brighter than the other RR Lyrae stars of M5 and are in an advanced evolutionary stage moving off from the HB redward. More than one-third of the M5 RR Lyrae stars investigated have irregular period change. The irregular period behaviour is relatively more frequent among the RRc (RR1) stars (50 per cent) than among the RRab (RR0) stars (34 per cent). A strict relationship has been found between the irregular period change and the Blazhko effect of M5 RRab stars. This fact indicates a common origin for these phenomena. It is remarkable that, if the RRab stars showing Blazhko effect are omitted from the sample, the mean rates of the period change have small positive values (〈β〉 = 0.012 ± 0.147 d Myr -1 , 〈α〉 = 0.013 ± 0.279 Myr -1 ), in excellent agreement with HB evolutionary model predictions.