RRd Stars Research Articles

A search for variable stars in the globular cluster M3

We describe here results of a photometric time-sequence survey of the globular cluster M3 (NGC 5272), in a search for contact and detached eclipsing binary stars. We have discovered only one likely eclipsing binary and one SX Phe type star in spite of monitoring 4077 stars with $V<20.0$ and observing 25 blue stragglers. The newly identified SX Phe star, V237, shows a light curve with a variable amplitude. Variable V238 shows variability either with a period of 0.49 d or with a period of 0.25 d. On the cluster colour-magnitude diagram, the variable occupies a position a few hundredths of magnitude to the blue of the base of the red giant branch. V238 is a likely descendent of a binary blue straggler. As a side result we obtained high quality data for 42 of the previously known RR Lyrae variables, including 33 of Bailey type ab, 7 type c and 2 double-mode pulsators. We used equations that relate the physical properties of RRc stars to their pulsation periods and Fourier parameters to derive masses, luminosities, temperatures and helium parameters for five of the RRc stars. One of the RRd stars (V79) has switched modes. In previous studies, it was classified as RRab, but our observations show that it is an RRd star with the first overtone mode dominating. This indicates blueward evolution on the horizontal branch.

Evolution of Horizontal-Branch Stars in Globular Clusters: The Interesting Case of V79 in M3

New observations of variable stars in the globular cluster M3 reveal that the RR Lyrae variable V79 is a double-mode (RRd) variable with the first overtone mode dominating. In all previous studies, V79 was found to be a fundamental mode (RRab) pulsator with an irregular light curve. This is the first observed mode switch for an RR Lyrae variable and it is direct observational evidence for blueward evolution of horizontal branch stars in the Oosterhoff type I cluster M3. It also demonstrates that there is a connection between the Blazhko effect and pulsational mode mixing in RR Lyrae variables. These new observations also show that the strength of the overtone oscillations in the RRd star V68 in M3 may have increased in the last 70 years, thus indicating blueward evolution for V68 as well. A survey of previously published investigations of RRd stars in Oosterhoff type II systems indicates that there is marginal evidence for an increase in the strength of fundamental mode oscillations in two stars: V30 in M15 and AQ Leo. If these increases are confirmed by future observations, it will indicate redward evolution for RRd stars in type II systems.

The MACHO Project Large Magellanic Cloud Variable Star Inventory. III. Multimode RR Lyrae Stars, Distance to the Large Magellanic Cloud, and Age of the Oldest Stars

We report the discovery of 73 double-mode RR Lyrae (RRd) stars in fields near the bar of the Large Magellanic Cloud (LMC). The stars are detected among the MACHO database of short-period variables that currently contains about 7900 RR Lyrae stars. Fundamental periods (P0) for these stars are found in the range 0.46-0.55 days, and first-overtone-to-fundamental period ratios are found to be in the range 0.742 < P1/P0 < 0.748. The range in period ratios is unexpectedly large, and a significant fraction of our current sample has period ratios smaller than any previously discovered RRd variables. We present mean magnitudes, colors, and light-curve properties for all LMC RRd stars detected to date. We undertake a determination of the absolute magnitudes for these stars based primarily on pulsation theory and on the assumption that all observed stars are at the fundamental blue edge of the instability strip. Comparison of the calibrated MACHO V and RKC photometry with these derived absolute magnitudes yields an absorption-corrected distance modulus to the LMC of 18.48 ± 0.19 mag that is in good agreement with that found (18.5) through comparison of Galactic and LMC Cepheids. Exploring this luminosity calibration, we derive an increase in the distance modulus, and thus a reduction in the age found via isochrone fitting, for M15 of about 30% and discuss some implications for cosmology.

CCD Photometry of Galactic Globular Clusters.III.IC 4499

view Abstract Citations (90) References (67) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS CCD Photometry of Galactic Globular Clusters.III.IC 4499 Walker, Alistair R. ; Nemec, James M. Abstract The variable star population of the Oosterhoff Type I galactic globular cluster IC 4499 (C1452-820) has been studied by CCD photometry, from observations in the B, V, R, and I passbands made during the period 1987-1994. Light curves have been prepared for 97 RR Lyrae variables and one variable blue-straggler (SX Phe) star. The RR Lyraes have <V>=17.65±0.01, and via several methods E(B-V)=0.22±0.02, with [Fe/H]=-1.65. The cluster contains a substantial number (17) of double-mode (RRd) variables, and period ratios are determined for 16 of these; it is near-constant, P1/P0=0/74432±0.00003 (s.e.). Calibrating using pulsation models by Cox, the mean mass is found to be 0.70±0.02 Msun and the Oosterhoff lI-Osterhoff I mass difference, comparing with similar observations of the M68 RRd stars, is 0.05±0.02 Msun. Four stars are identified as second-overtone (RRe) pulsators. The distribution of stars on the horizontal branch (HB) is compared with predictions from evolution and pulsation theory. Publication: The Astronomical Journal Pub Date: November 1996 DOI: 10.1086/118161 Bibcode: 1996AJ....112.2026W Keywords: GLOBULAR CLUSTERS: INDIVIDUAL: IC 4499 full text sources ADS | data products SIMBAD (6) Related Materials (5) Part 1: 1992PASP..104.1063W Part 2: 1994AJ....108..555W Part 4: 1998AJ....116..220W Part 5: 1999AJ....118..432W Part 6: 2000AJ....119.1512W

The RR Lyrae Variable Stars in the Globular Cluster M15

view Abstract Citations (71) References (63) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The RR Lyrae Variable Stars in the Globular Cluster M15 Silbermann, N. A. ; Smith, Horace A. Abstract New CCD photometry is presented for RR Lyrae variable stars in the globular cluster M15. This photometry, mainly in V and R, with some additional B and I, is used to construct lightcurves of 44 RR Lyraes and one Cepheid in M15. One new low-amplitude RR Lyrae star was discovered. The observable parameters of the light curves are used, in conjunction with theoretical photometric indices, to obtain effective temperatures and bolometric magnitudes for the RR Lyrae stars. We confirm that the regions occupied by RRab and RRcd variables in M15 have little or no overlap in effective temperature. Photometry for variable and nonvariable horizontal branch stars in Ml5 is used to establish the high and low temperature boundaries of the instability strip. There is evidence that the blue edge of the M15 RR Lyrae instability strip becomes cooler at higher luminosities. Employing Cox's value of the mass of RRd stars, 0.75 Msun, and the pulsation equation, the absolute magnitude of the RR Lyrae stars in M15 is estimated to be <MV>= +0.36±0.12 mag. However, this result depends upon the adopted mass, bolometric correction, and color-temperature relation, which remain significantly uncertain. The implications of the absolute magnitude determination for the ages of the globular clusters are discussed. Publication: The Astronomical Journal Pub Date: August 1995 DOI: 10.1086/117555 Bibcode: 1995AJ....110..704S Keywords: STARS: VARIABLES: RR LYRAE; GLOBULAR CLUSTERS: INDIVIDUAL: M15 full text sources ADS | data products SIMBAD (82) CDS (1) Related Materials (1) Erratum: 1996AJ....111..567S

Comparative pulsation calculations with OP and OPAL opacities

Comparative linear nonadiabatic pulsation calculations are presented using the OPAL and Opacity Project opacities. The two sets of opacities include effects due to intermediate coupling and fine structure as well as new abundances. We used two mass luminosity (M-L) relations, one standard (BIT), and one employing substantial convective core overshoot (COV). The two sets of opacities cannot be differentiated on the basis of the stellar pulsation calculations presented here. The BIT relation can model the beat and bump Cepheids with masses between 4 and 7 solar mass, while if the overshoot relation is used, masses between 2 and 6 solar mass are required. In the RR Lyrae regime, we find the inferred masses of globular cluster RRd stars to be little influenced by the choice of OPAL or OP. Finally, the limited modeling we have done is not able to constrain the Cepheid M-L relation based upon period ratios observed in the beat and bump stars.

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.

Double-mode RR Lyrae models

AbstractDouble-mode RR Lyrae stars (RRd) represent a fundamental testing ground for the theory of stellar pulsation because they play an important role in understanding the interaction between different pulsation modes. In spite of the fact that from an observational point of view the identification of the fundamental (F) and first overtone (IO) periods needs only few periods of light curve coverage (Sandage et al. 1981, Nemec 1985), the theoretical models of double-mode and mixed-mode variables dating back to the original papers of Cox et al. (1983) and Stellingwerf (1975) were not integrated for long enough times to ensure proper mode identification. Recently Kovacs and Buchler (1992) using a standard radiative hydrodynamical code investigated the theoretical properties of RRd stars in more detail. This paper represents a brief description of the results of few cases computed adopting a non-linear, non-local treatment of convective transport, and long integration times to enable positive mode identification.

Double-Mode Stellar Pulsation

AbstractThe current observational and theoretical status of the double-mode variables is reviewed. Focusing mostly on the RR Lyrae stars, we address the question of the observational evidence of modal stability. The problem of stationarity is a crucial issue in the modelling of these stars.We mention past efforts in hydrodynamical and analytical modelling together with a detailed discussion of some very recent results. It is suggested that stochastic forcing due to turbulent convection may play a crucial role in exciting some marginally stable modes in the limiting pulsation. The latest hydrodynamical results first demonstrate that purely radiative models are able to show permanent double-mode behavior in the relevant period regime of RRd stars. The reason for the previous lack of double-mode behavior is attributed to the large dissipation, i.e. artificial viscosity, generally used in the codes to ensure numerical stability and to obtain amplitudes comparable to the observed ones.We think that better models should include some physical dissipation, most probably turbulent convection, and a more accurate numerical treatment of the radiative hydrodynamics.

The LMC cluster GLC 0435 - 59 (Reticulum) - Photometry of the RR Lyraes, and a color-magnitude diagram

B and V CCD photometry has been used to study the RR Lyraes and to prepare a color-magnitude diagram for the outlying Large Magellanic Cloud (LMC) cluster GLC 0435-59 (the Reticulum system). Ten new variables have been found bringing the total known to 32. One possible RRd star has been identified. The pulsation properties of the variables indicate [Fe/H]=−1.7±0.1, while the reddening is E(B−V)=0.03±0.02. The dispersion of mean magnitudes of the RR Lyraes is extremely small (∼0.1 mag) and is interpreted as signifying that all the variables are still very close to the ZAHB.

Two double-mode RR Lyrae stars in the field

In order to use RR Lyrae stars as reliable distance indicators, it is necessary to know how the masses of these stars are related to the metal abundance. RR Lyrae stars in the field have a greater range of periods and metal abundance than those in globular clusters. Thus, it is important to identifying double-mode RR Lyrae stars (RRd stars) in the field to test the mass-metal abundance relation. Up to the present, however, the only RRd star to have been identified in the field is AQ Leo. Here, evidence is presented for the existence of two others. Data are given on their periods, mean B magnitudes, mode amplitudes, mean B-V colors, masses, and (Fe/H) values. The results give no evidence for a mass-metallicity relationship among the three known RRd field stars, but they show overall agreement with the mass-metallicity relation given by cluster RRd stars. 34 refs.

A search for double-mode RR Lyrae stars in the globular clusters M80, M9, and NGC 2298

view Abstract Citations (17) References (41) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Search for Double-Mode RR Lyrae Stars in the Globular Clusters M80, M9, and NGC 2298 Clement, Christine M. ; Walker, Ian R. Abstract Observations of RR Lyrae variables near the transition between fundamental and first-overtone mode in the Oosterhoff type II (Oo II) globular clusters M80, M9, and NGC 2298 have been analyzed in a search for double-mode pulsators (RRd stars). These three clusters were selected because they have metal abundances that are intermediate between those of the Oo II clusters and the Oo I clusters that are known to have RRd stars. Any RRd stars identified in these three clusters would be useful for verification of a mass-metallicity relation. The analysis was based on published observations for all three clusters, and in addition, 44 CCD frames of M9 and 32 photographs of NGC 2298, all obtained with the University of Toronto telescope at Las Campanas. No new RRd stars were discovered. It is noted that the only globular clusters known to have RRd stars are the most metal poor Oo I clusters and metal poor Oo II clusters. Published observations of the RRd stars in M15 have been analyzed in order to determine accurate periods and period ratios for stars whose periods were previously considered to be uncertain. Publication: The Astronomical Journal Pub Date: April 1991 DOI: 10.1086/115771 Bibcode: 1991AJ....101.1352C Keywords: Blue Stars; Cepheid Variables; Giant Stars; Globular Clusters; Stellar Oscillations; Abundance; Charge Coupled Devices; Light Curve; Metallicity; Stellar Mass; Astronomy; CLUSTERS: GLOBULAR; STARS: RR LYRAE full text sources ADS | data products SIMBAD (14) CDS (1)

On Omega Centauri and its RRc stars

view Abstract Citations (17) References (34) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On omega Centauri and Its RRc Stars Simon, Norman R. Abstract Masses, absolute temperatures, and luminosities are derived for a large sample of RRc stars in the globular cluster Omega Centauri, interpreting the observed periods, relative luminosities, and Fourier phases in terms of linear and hydrodynamic pulsation models. The resulting masses agree with those of the RRd stars in other clusters and the derived parameters pass a number of other tests which are independent of their derivation. The results disagree with current horizontal branch models and also imply that the canonical slope of the log T(e) versus B-V relation is too steep by a factor of two. It is shown that the disagreement between the RRd masses and those based on horizontal branch calculations cannot be reconciled by reasonable changes in metallicity or opacity. No correlation between the masses of the Omega Cen RRc stars and their metallicities is found, and it is shown that this cannot be due to the late redward evolution by the RR Lyrae stars of lower mass. Publication: The Astrophysical Journal Pub Date: September 1990 DOI: 10.1086/169101 Bibcode: 1990ApJ...360..119S Keywords: Giant Stars; Globular Clusters; Stellar Models; Stellar Physics; Variable Stars; Blue Stars; Canonical Forms; Centaurus Constellation; Stellar Mass; Stellar Oscillations; Astrophysics; CLUSTERS: GLOBULAR; STARS: ABUNDANCES; STARS: PULSATION; STARS: RR LYRAE full text sources ADS | data products SIMBAD (10)

A search for double-mode RR Lyrae stars in the globular cluster M68

Published observations of 23 c-type RR Lyr stars in M68 have been analyzed in a search for double-mode pulsators (RRd stars). There are at least seven and possibly as many as 10 RRd stars with primary periods in the range of 0.39-0.41 d. Reliable values of both the fundamental and first overtone periods have been obtained for one of the stars (V3). The double-mode pulsation mass for V3, using the King Ia (Y = 0.299, Z = 0.001) mass calibration of Cox et al. (1983), is 0.64 + or - 0.005 solar mass.

A search for double-mode RR Lyrae stars in M3

Photometric observations obtained between 1895 and 1962 are used to systematically search for double mode RR Lyrae (RRd) stars in M3. Of the 46 stars considered, only V68 and V87 (both of which were previously identified as double-mode pulsators) are shown to be certain RRd variables. Masses of 0.52 + or - 0.005 solar masses for V68 and 0.55 + or - 0.015 solar masses for V87 are obtained, confirming the suggestion that RRd stars in Oosterhoff Type I systems are systematically 20 percent less massive, and correspondingly less luminous, than RRd stars in Oosterhoff Type II systems. Although evidence is found for cycle-to-cycle light variations, no significant long-term amplitude changes were noted. 61 refs.

On the masses and luminosities of RR Lyrae stars

Some results are given from the comparison with observations of a large number of hydrodynamic pulsation models. It is found that linear pulsation calculations are adequate for the determination of the masses of RRd stars, that the Fourier phase parameter phi31 measures the luminosity-to-mass ratio L/M exp 1.81 for RRc stars, that most of the Omega Cen RRc stars seem to be evolved, as has previously been suggested, that the masses of the Omega Cen RRc stars agree with the masses of RRd stars in other clusters; and a mass-helium abundance relation may exist among the RRc stars in Omega Cen.

Double-Mode RR Lyrae Stars in IC 4499

Thirteen double-mode RR Lyrae (RRd) stars, with mean magnitudes c = 18.30 ± 0.10 and &lt;V&gt;c = 17.80 ± 0.15, have been identified in the variable-rich Oosterhoff type I globular cluster IC 4499. The stars have surprisingly uniform properties, and are considerably different from RRd stars found in Oo II systems. The mean first-overtone period (Fig.1) is &lt;P1&gt;=0.357d ± 0.005d (cf. &lt;P1&gt;=0.40d for Oo II RRd stars), and the mean ratio of the first-overtone period to the fundamental period is &lt;P1/P0&gt;= 0.7443 ± 0.0002. The mean double-mode pulsation mass for the 13 stars, using the King Ia (Y=0.279, Z=0.001) mass calibration, is 0.535 ± 0.003 M⊙. Such an average mass is 0.11 M⊙ smaller (i.e. ~17% smaller) than that for RRd stars found in Oo II systems, and possibly ~0.01 M⊙smaller than the mean mass for the two RRd stars in M3 (it is important to note that the zero point of these mass determinations is uncertain by at least 15%). The metal abundances for the RRd stars, and for the system of RR Lyrae stars as a whole, are found to be consistent with &lt;[Fe/H]&gt;=-1.38 ± 0.20, determined from δS spectroscopy. In the Peterson diagram (Fig.2), all known RRd stars now divide (apparently by mass) into two groups (split according to Oosterhoff type). With a reddening of EB-V=0.26 ± 0.03, the cluster distance modulus is (m-M)o=16.23m ± 0.23m.

A search for double-mode RR Lyrae stars in Omega Centauri

Using Martin's (1939) extensive photometry of variable stars in the globular cluster Omega Centauri (NGC 5139 = C1323-472), the brightness variations of 55 RR Lyrae stars have been thoroughly analyzed in a search for multiply periodic stars. Despite the fact that there are at least six short-period ab type Blazhko variables in Omega Cen, in the period interval 0.35 to 0.50 days, there appear to be no doubly periodic RR Lyrae stars of the kind previously found in the globular clusters M3, M15, M68, and IC 4499, and in the Draco dwarf galaxy. One possible reason for the absence of such stars in Omega Cen is that the horizontal-branch stars might be rotating relatively faster than similar stars in systems known to have produced RRd stars. This might also explain the apparent absence of RRd stars in M5 (Nemec and Clement, 1986). Other possibilities are discussed. 61 references.

Double-mode RR Lyrae stars in M15 - Reanalysis, and experiments with simulated photometry

A new analysis of the periods and amplitudes of 12 double-mode RR Lyrae stars in M15 is presented. The results are in excellent agreement with those derived by Cox, Hodson, and Clancy (1983) from the same data using Fourier series techniques. The mean beat mass for 11 stars is 0.658 + or 0.004 (internal standard error) + pr - 0.05 (estimated probable uncertainty). Uncertainties are discussed and light curves are presented for all the stars. Experiments are described that show the effects of random scatter on the derived periods of the RRd stars in M15 and Draco. They provide evidence that the detected secondary oscillations probably are real.