Barton J. Pritzl

A Comparison of Elemental Abundance Ratios in Globular Clusters, Field Stars, and Dwarf Spheroidal Galaxies

We have compiled a sample of globular clusters with high-quality stellar abundances from the literature to compare to the chemistries of stars in the Galaxy and in dwarf spheroidal galaxies. Of the 45 globular clusters examined, 29 also have kinematic information. Most of the globular clusters belong to the Galactic halo; however, a significant number have disk kinematics or belong to the bulge. Focusing on the [α/Fe] and light r-process element ratios, we find that most globular cluster stars mimic field stars of similar metallicities, and neither clearly resembles the currently available stellar abundances in dwarf galaxies (including globular clusters in the Large Magellanic Cloud). The exceptions to these general elemental ratio comparisons are already known in the literature, e.g., ω Centauri, Palomar 12, and Terzan 7 associated with the Sagittarius remnant and Ruprecht 106, which has a high radial velocity and low [α/Fe] ratio. A few other globular clusters show more marginal peculiarities. The most notable one is the halo cluster M68, which has a high galactocentric rotational velocity, a slightly younger age, and a unique [Si/Ti] ratio. The [Si/Ti] ratios decrease with increasing [Fe/H] at intermediate metallicities, which is consistent with very massive stars playing a larger role in the early chemical evolution of the Galaxy. The chemical similarities between globular clusters and field stars with [Fe/H] ≤ -1.0 suggests a shared chemical history in a well-mixed early Galaxy. The differences in the published chemistries of stars in the dwarf spheroidal galaxies suggest that neither the globular clusters, halo stars, nor thick disk stars had their origins in small isolated systems like the present-day Milky Way dwarf satellites.

Optical Light Curve of the Type Ia Supernova 1998bu in M96 and the Supernova Calibration of the Hubble Constant

We present the UBV(RI)KC light curves of the Type Ia supernova SN 1998bu, which appeared in the nearby galaxy M96 (NGC 3368). M96 is a spiral galaxy in the Leo I group that has a Cepheid-based distance. Our photometry allows us to calculate the absolute magnitude and reddening of this supernova. These data, when combined with measurements of the four other well-observed supernovae with Cepheid-based distances, allow us to calculate the Hubble constant with respect to the Hubble flow defined by the distant Calan/Tololo Type Ia sample. We find a Hubble constant of 63.9 ± 2.2(internal) ± 3.5(external) km s-1 Mpc-1, consistent with most previous estimates based on Type Ia supernovae. We note that the two well-observed Type Ia supernovae in Fornax, if placed at the Cepheid distance to the possible Fornax spiral NGC 1365, are apparently too faint with respect to the Calan/Tololo sample calibrated with the five Type Ia supernovae with Cepheid distances to the host galaxies.

Variable stars in the unusual, metal-rich globular cluster NGC 6441

We have undertaken a search for variable stars in the metal-rich globular cluster NGC 6388 using time-series BV photometry. Twenty-eight new variables were found in this survey, increasing the total number of variables found near NGC 6388 to approx. 57. A significant number of the variables are RR Lyrae (approx. 14), most of which are probable cluster members. The periods of the fundamental mode RR Lyrae are shown to be unusually long compared to metal-rich field stars. The existence of these long period RRab stars suggests that the horizontal branch of NGC 6388 is unusually bright. This implies that the metallicity-luminosity relationship for RR Lyrae stars is not universal if the RR Lyrae in NGC 6388 are indeed metal-rich. We consider the alternative possibility that the stars in NGC 6388 may span a range in [Fe/H]. Four candidate Population II Cepheids were also found. If they are members of the cluster, NGC 6388 would be the most metal-rich globular cluster to contain Population II Cepheids. The mean V magnitude of the RR Lyrae is found to be 16.85 +/- 0.05 resulting in a distance of 9.0 to 10.3 kpc, for a range of assumed values of (M(sub V)) for RR Lyrae. We determine the reddening of the cluster to be E(B - V) = 0.40 +/- 0.03 mag, with differential reddening across the face of the cluster. We discuss the difficulty in determining the Oosterhoff classification of NGC 6388 and NGC 6441 due to the unusual nature of their RR Lyrae, and address evolutionary constraints on a recent suggestion that they are of Oosterhoff type II.

Hubble Space Telescope Snapshot Study of Variable Stars in Globular Clusters: The Inner Region of NGC 6441*

We present the results of a Hubble Space Telescope snapshot program to survey the inner region of the metal-rich globular cluster NGC 6441 for its variable stars. A total of 57 variable stars were found, including 38 RR Lyrae stars, six Population II Cepheids, and 12 long-period variables. Twenty-four of the RR Lyrae stars and all of the Population II Cepheids were previously undiscovered in ground-based surveys. Of the RR Lyrae stars observed in this survey, 26 are pulsating in the fundamental mode with a mean period of 0.753 days and 12 are first-overtone–mode pulsators with a mean period of 0.365 days. These values match up very well with those found in ground-based surveys. Combining all the available data for NGC 6441, we find mean periods of 0.759 and 0.375 days for the RRab and RRc stars, respectively. We also find that the RR Lyrae stars in this survey are located in the same regions of a period-amplitude diagram as those found in ground-based surveys. The overall ratio of RRc to total RR Lyrae stars is 0.33. Although NGC 6441 is a metal-rich globular cluster and would, on that ground, be expected either to have few RR Lyrae stars or to be an Oosterhoff type I system, its RR Lyrae stars more closely resemble those in Oosterhoff type II globular clusters. However, even compared with typical Oosterhoff type II systems, the mean period of its RRab stars is unusually long. We also derived I-band period-luminosity relations for the RR Lyrae stars. Of the six Population II Cepheids, five are of W Virginis type and one is a BL Herculis variable star. This makes NGC 6441, along with NGC 6388, the most metal-rich globular cluster known to contain these types of variable stars. Another variable, V118, may also be a Population II Cepheid, given its long period and its separation in magnitude from the RR Lyrae stars. We examine the period-luminosity relation for these Population II Cepheids and compare it with those in other globular clusters and in the Large Magellanic Cloud. We argue that there does not appear to be a change in the period-luminosity relation slope between the BL Herculis and W Virginis stars, but that a change of slope does occur when the RV Tauri stars are added to the period-luminosity relation.

The Dwarf Spheroidal Companions to M31: Variable Stars in Andromeda II

We present the results of a variable star search in Andromeda II, a dwarf spheroidal galaxy companion to M31, using Hubble Space Telescope Wide Field Planetary Camera 2 observations. Seventy-three variables were found, one of which is an anomalous Cepheid while the others are RR Lyrae stars. The anomalous Cepheid has properties consistent with those found in other dwarf spheroidal galaxies. For the RR Lyrae stars, the mean periods are 0.571 and 0.363 days for the fundamental mode and first-overtone mode stars, respectively. With this fundamental mode mean period and the mean metallicity determined from the red giant branch ([Fe/H] = -1.49), Andromeda II follows the period-metallicity relation defined by the Galactic globular clusters and other dwarf spheroidal galaxies. We also find that the properties of the RR Lyrae stars themselves indicate a mean abundance that is consistent with that determined from the red giants. There is, however, a significant spread among the RR Lyrae stars in the period-amplitude diagram, which is possibly related to the metallicity spread in Andromeda II indicated by the width of the red giant branch, which was determined by Da Costa et al. In addition, the abundance distribution of the RR Lyrae stars is notably wider than the distribution expected from the abundance determination errors alone. The mean magnitude of the RR Lyrae stars, VRR = 24.87 ± 0.03, implies a distance d = 665 ± 20 kpc to Andromeda II. This matches the distance derived from the mean magnitude of the horizontal branch stars by Da Costa et al., d = 680 ± 20 kpc. We also demonstrate that the specific frequency of anomalous Cepheids in dwarf spheroidal galaxies correlates with the mean metallicity of their parent galaxy and that the Andromeda II and Andromeda VI anomalous Cepheids appear to follow the same relation as those in the Galactic dwarf spheroidal galaxies.

RR Lyrae Stars in NGC 6388 and NGC 6441: A New Oosterhoff Group?

NGC 6388 and NGC 6441 are anomalies among Galactic globular clusters in that they cannot be readily placed into either Oosterhoff group I or Oosterhoff group II despite their significant numbers of RR Lyrae variables. The mean pulsation periods, &angl0;Pab&angr0;, of their RRab variables, at 0.71 and 0.76 days, respectively, are even larger than for Oosterhoff II clusters. Moreover, Oosterhoff II clusters are very metal poor, whereas NGC 6388 and NGC 6441 are the most metal-rich globular clusters known to contain RR Lyrae stars. The location of the NGC 6388 and NGC 6441 RRab variables in the period-amplitude diagram implies that the RR Lyrae stars in those two clusters are brighter than expected for their metallicities. Our results therefore indicate that a universal relationship may not exist between the luminosity and the metallicity of RR Lyrae variables.

THE CHANGING BLAZHKO EFFECT OF XZ CYGNI

New CCD photometry has been obtained for the RR Lyrae variable star XZ Cygni. An analysis of old and new photometry confirms earlier results that XZ Cyg exhibits the Blazhko effect and that its Blazhko period has changed over time. These changes in the Blazhko period are anticorrelated with observed changes in the primary period of XZ Cyg. During the first half of the 20th century, XZ Cyg had a Blazhko period of approximately 57.4 days. Beginning in 1965, its primary period underwent a steep decline in several steps. Coincidentally, its Blazhko period increased to about 58.5 days. In 1979, the primary period suddenly increased again. After an interval in which the Blazhko effect was small, the Blazhko effect reestablished itself, with a period of approximately 57.5 days. When its Blazhko period is near 57.5 days, XZ Cyg has also shown a tertiary period of 41.6 days. We confirm that there is evidence for a longer 3540 day period in photometry obtained during the first half of the 20th century. XZ Cyg is compared with three other RR Lyrae stars that also appear to show changing Blazhko periods. The observed changes in the length of the Blazhko period of XZ Cyg constrain possible explanations for the Blazhko effect. In particular, they argue against any theoretical explanation that requires that the Blazhko period be exactly equal, or directly proportional, to the rotation period of the star.

The Dwarf Spheroidal Companions to M31: Variable Stars in Andromeda I and Andromeda III*

We present the results of variable star searches of the M31 dwarf spheroidal (dSph) companions Andromeda I and Andromeda III using the Hubble Space Telescope. A total of 100 variable stars were found in And I, while 56 were found in And III. One variable found in And I and another in And III may be type II Cepheids. In addition to this variable in And III, another four variables are anomalous Cepheids (ACs). So far, no definite ACs have been discovered in And I. We discuss the properties of these variables with respect to those found in the other dSph galaxies and revisit the AC period-luminosity relations. We found 72 fundamental mode RR Lyrae (RRab) stars and 26 first-overtone mode RR Lyrae (RRc) stars in And I giving mean periods of 0.575 and 0.388 days, respectively. One likely RR Lyrae star in And I remains unclassified because of a lack of F555W data. For And III, 39 RR Lyrae stars are pulsating in the fundamental mode with a mean period of 0.657 days, and 12 are in the first-overtone mode with a mean period of 0.402 days. Using the mean metal abundances derived from the red giant branch colors, the mean RRab period for And I is consistent with the mean period-metallicity relation seen in the RR Lyrae populations of Galactic globular clusters, while And III is not, having too large a mean RRab period for its abundance. In And I, we found two RR Lyrae stars that are noticeably fainter than the horizontal branch. We discuss the possibility that these stars are associated with the recently discovered stellar stream in the halo of M31. Using various methods, we estimate the mean metallicity of the RR Lyrae stars to be [Fe/H] ≈ -1.5 for And I and ≈-1.8 for And III. These estimates match well with other mean metallicity estimates for the galaxies. Interestingly, a comparison of the period-amplitude diagrams for these two galaxies with other dSph galaxies shows that And III is lacking in shorter period, higher amplitude RR Lyrae stars. This may be a consequence of the very red horizontal branch morphology in this dSph. Not including the two faint RR Lyrae stars, we find VRR = 25.14 ± 0.04 mag for And I, resulting in a distance of 765 ± 25 kpc. For And III, VRR = 25.01 ± 0.04 mag, giving a distance of 740 ± 20 kpc. These distance estimates are consistent with those previously found for these galaxies. We discuss the relation between the specific frequency of the ACs in dSph galaxies and the mean metallicity of the galaxy, finding that the M31 dSph galaxies follow the same relations as the Galactic dSph galaxies. We also find that the period-luminosity relations of ACs and short-period Cepheids are distinct, with the short-period Cepheids having higher luminosities at a given period.

Variable Stars in the Newly Discovered Milky Way Dwarf Spheroidal Satellite Canes Venatici I

We have identified 23 RR Lyrae stars and three possible anomalous Cepheids (ACs) among 84 candidate variables in the recently discovered Canes Venatici I (CVn I) dwarf spheroidal galaxy. The mean period of 18 RRab type stars, --> Pab = 0.60 ± 0.01 days, and the location of these stars in the period-amplitude diagram suggest that CVn I is likely an Oosterhoff-intermediate system. The average apparent magnitude of the RR Lyrae stars, --> V = 22.17 ± 0.02 mag, gives a distance of 210 -->+ 7−5 kpc, for an adopted reddening -->E(B − V) = 0.03 mag. We present a B, V color-magnitude diagram (CMD) of CVn I that reaches -->V ~ 25 mag, and shows that the galaxy has a mainly old stellar population with a metal abundance near [Fe/H] = –2.0 dex. The width of the red giant branch and the location of the candidate ACs on the CMD may indicate that the galaxy hosts a complex stellar population with stars from ~13 Gyr to as young as ~0.6 Gyr.

On the Newly Discovered Canes Venatici II dSph Galaxy

We report on the detection of variable stars in the Canes Venatici II (CVn II) dwarf spheroidal galaxy, a new satellite of the Milky Way recently discovered by the Sloan Digital Sky Survey. We also present a V, -->B − V color-magnitude diagram that reaches -->V ~ 25.5 mag, showing the galaxys main-sequence turnoff at -->V ~ 24.5 mag and revealing several candidate blue straggler stars. Two RR Lyrae stars have been identified within the half-light radius of CVn II, a fundamental-mode variable (RRab) with period -->Pab = 0.743 days, and a first-overtone (RRc) RR Lyrae star with -->Pc = 0.358 days. The rather long periods of these variables along with their position on the period-amplitude diagram support an Oosterhoff type II classification for CVn II. The average apparent magnitude of the RR Lyrae stars, --> V = 21.48 ± 0.02 mag, is used to obtain a precision distance modulus of -->μ0 = 21.02 ± 0.06 mag and a corresponding distance of -->160+ 4−5 kpc, for an adopted reddening -->E(B − V) = 0.015 mag.