Edward Schmidt Publications

The Double-Mode Cepheid V371 Persei Redux

Edward G. Schmidt — Tue, 08 Nov 2011 12:30:14 PST

A metallicity of [Fe/H] = –0.05 to –0.40 was derived from four spectra of the peculiar beat Cepheid V371 Per. This conflicts with previous estimates ranging from –0.7 to –1.0 based on the period ratio. It is suggested that the discrepancy may be resolved by assuming near solar metallicity but a low mass. This is in accord with a suggestion by Sandage and Tammann that the AHB3 (or BL Her) stars are Population I analogs of short-period type II Cepheids. Further pulsational modeling is needed to clarify the status of this star and to understand the AHB3 stars.

SPECTRA OF TYPE II CEPHEID CANDIDATES AND RELATED STARS

Edward G. Schmidt et al. — Thu, 27 Jan 2011 14:41:24 PST

We present low-resolution spectra for variable stars in the Cepheid period range from the ROTSE-I Demonstration Project and the All Sky Automated Survey, some of which were previously identified as type II Cepheid candidates. We have derived effective temperatures, gravities, and metallicities from the spectra. Based on this, three types of variables were identified: Cepheid strip stars, cool stars that lie along the red subgiant and giant branch, and cool main-sequence stars. Many fewer type II Cepheids were found than expected and most have amplitudes less than 0.4 mag. The cool variables include many likely binaries as well as intrinsic variables. Variation among the main-sequence stars is likely to be mostly due to binarity or stellar activity

Four-Color and Hβ Photometry of Stars in NGC 7654 and M25

Edward G. Schmidt — Wed, 31 Mar 2010 14:54:47 PDT

Four-color and Hβ photometry has been obtained of stars in the galactic clusters NGC 7654 and M25. An analysis of these data shows variable extinction in both clusters. The distance modnlus of NGC 7654 is found to be 10^m.99 ± 0.2 and that of M25 is 8^m.68 ± 0.1.

The Cepheid Luminosity Scale

Edward G. Schmidt — Wed, 31 Mar 2010 14:49:48 PDT

The distance moduli of galactic clusters containing classical Cepheids are being redetermined using the four-color and Hβ photometric system. The results for four clusters are presented here and it is found that the distance moduli are smaller than previous values. Possible reasons for this discrepancy are discussed.

Four-Color and Hβ Photometry of the Galactic Cluster NGC 6633

Edward G. Schmidt — Wed, 31 Mar 2010 14:41:21 PDT

Photometric measurements of stars in the intermediate-age galactic cluster NGC 6633 are presented. From these data the color excess is found to vary slightly across the cluster with an average of E_{b – y} = 0^m.124. The distance modulus is found to be 7^m.711 and the turnoff occurs at about (b – y)₀ = 0^m.l. The cluster is slightly metal poor.

Simultaneous X-Ray and Optical Observations of AN Ursae Majoris during a Low State

Paula Szkody et al. — Wed, 31 Mar 2010 14:33:42 PDT

X-ray observations throughout the orbital cycle of AN UMa during its low state were conducted with the Einstein Observatory simultaneously with ground-based optical UBV photometry and spectroscopy. The observed low X-ray flux of 7 × 10^–13 ergs cm^–2 s^–1 is a factor of 40 below the high-state X-ray flux, and F_x/F_opt is about a factor of 50 lower than for AM Her and 2A 0311–227. There is marginal evidence for an X-ray eclipse coincident with an optical minimum and for increased X-ray flux at magnetic phase 0.4 which does not correlate with any optical continuum variation but may be related to increased hydrogen and helium line emission fluxes. Subject headings: stars: individual - X-rays: binaries

Four-Color and Hβ Photometry of the Galactic Cluster NGC 7790

Edward G. Schmidt — Wed, 31 Mar 2010 14:22:19 PDT

Observations have been obtained on the four-color and Hβ system of 16 stars in the field of NGC 7790. These represent virtually all of the stars which are in the B-star region of the cluster color-magnitude diagram. I discuss the membership of these stars and find three which are possibly field stars. The remaining stars indicate a true distance modulus of 11.98 ± 0.13 for the cluster and a color excess of E(b – y) = 0.403.

Four-Color and Hβ Photometry of Stars in the Galactic Cluster NGC 6087

Edward G. Schmidt — Wed, 31 Mar 2010 14:18:34 PDT

Four-color and Hβ photometry has been obtained for 16 stars in the open cluster NGC 6087. Based on this photometry the membership of these stars in the cluster is discussed. Using stars which appear to be members a color excess of E(b – y) = 0^m.23 and a distance modulus of (V₀ – M_v) = 9^m.60 are derived. This distance modulus is about 0^m.4 less than previous values and indicates a lower luminosity for the Cepheid member, S Nor, than is currently accepted.

Four-Color And Hβ Photometry of Stars in the Galactic Cluster NGC 129

Edward G. Schmidt — Wed, 31 Mar 2010 14:11:48 PDT

Four-color and photometry has been obtained for 22 stars in the field of the open cluster NGC 129. Based on this photometry, the membership of these stars in the cluster is discussed and it is found that many are field stars. The color excesses for cluster members obtained here range from E(b – y) = 0.207 to 0.513 and are larger than those found from previous UBV photometry. It has been suggested that this cluster contains a nonvariable Cepheid strip star. However, the present reddening places this star at the edge of the instability strip. A true distance modulus of 10.93 mag is found for this cluster, which is less than some previous values.

Mining Sky Surveys for Astrophysically Interesting Variable Stars: The Cepheid Period Range

Edward G. Schmidt — Mon, 29 Mar 2010 14:45:26 PDT

We have undertaken a project to discover new type II Cepheids in two large-area sky surveys. However, most of the candidates from the surveys differ from type II Cepheids in their amplitude and period distributions and they form a separate, cooler sequence relative to the Cepheids in a temperature-gravity plot. We refer to these objects as "Cepheid-lke" stars. Many exhibit unstable pulsation which we illustrate with representative light curves.

Intermediate-Band Photometry of the Open Cluster NGC 3114

Edward G. Schmidt — Thu, 25 Mar 2010 12:57:04 PDT

Four-color and Hβ photometry has been obtained of 33 stars in the southern galactic cluster NGC 3114. This cluster contains many interesting stars including blue stragglers, red giants, and stars with spectral peculiarities. The distance modulus was found to be V₀ – M_v = 10.26 which corresponds to 1130 pc. The color excess is E(b – y) = 0^m.060. The cluster appears to be slightly older than the Pleiades. There are two extreme blue stragglers and six additional stars which might be considered milder cases of the same phenomenon. The distance moduli of the stars with peculiar spectra and the blue stragglers inferred from the photometry agree with those of the normal cluster stars and we conclude that this strengthens the view that the photometric indices of such stars are not affected significantly by their peculiarities.

Four-Color and Hβ Photometry of the Galactic Cluster M25

Edward G. Schmidt — Thu, 25 Mar 2010 12:48:26 PDT

Four-color and Hβ photometry has been obtained for 39 stars in the field of the galactic cluster M25. The membership of the individual stars is discussed. From 27 stars which are considered likely members we obtain a mean distance modulus of 8.76 and a mean color excess of E(b – y) = 0.341.

Intermediate-Band Photometry in the Open Cluster NGC 6664

Edward G. Schmidt — Thu, 25 Mar 2010 12:45:15 PDT

Four-color and Hβ photometry is given for early-type stars in the open cluster NGC 6664. From these data the true distance modulus is found to be 10.7 and the reddening is found to range from E (b – y) = 0.48 to 0.63 across the cluster. The distance modulus agrees well with some values from the literature, but the present color excesses are about 0.1 mag larger than published values. There are two or perhaps three blue stragglers in the cluster and its age is estimated to be 46 × 10⁶ yr.

Intermediate-Band Photometry of Stars in Three Clusters Containing Classical Cepheids

Edward G. Schmidt — Thu, 25 Mar 2010 12:40:55 PDT

Four-color and Hβ photometry has been carried out of stars in three sparse and distant clusters which are thought to contain Cepheids. From these data the distance moduli and color excesses of the clusters have been derived. In general the new values agree with those in the literature but are somewhat more precise.

HR Aurigae: An Active Binary ?

Charles G. Loomis et al. — Thu, 25 Mar 2010 12:36:23 PDT

Photometric and spectral observations of the variable star HR Aurigae are presented. Based on these data we conclude that this star, previously regarded as a type II Cepheid, is, in fact, an active binary.

V371 Per — A thick-disk, short-period F/1O Cepheid

P. Wils et al. — Wed, 24 Mar 2010 12:25:04 PDT

V371 Per was found to be a double-mode Cepheid with a fundamental mode period of 1.738 days, the shortest among Galactic beat Cepheids, and an unusually high period ratio of 0.731, while the other Galactic beat Cepheids have period ratios between 0.697 and 0.713. The latter suggests that the star has a metallicity [Fe/H] between −1 and −0.7. The derived distance from the Galactic plane places it in the thick disk or the Halo, while all other Galactic beat Cepheids belong to the thin disk. There are indications from historical data that both the fundamental and first overtone periods have lengthened.

PHOTOMETRY OF TYPE II CEPHEID CANDIDATES FROM THE NORTHERN PART OF THE ALL SKY AUTOMATED SURVEY

Edward G. Schmidt et al. — Fri, 22 May 2009 09:22:06 PDT

We have obtained VR photometry of 282 Cepheid variable star candidates from the northern part of the All Sky Automated Survey (ASAS). These together with data from the ASAS and the Northern Sky Variability Survey (NSVS) were used to redetermine the periods of the stars. We divided the stars into four groups based on location in a plot of mean color, (V − R), versus period. Two of the groups fell within the region of the diagram containing known type II Cepheids and yielded 14 new highly probable type II Cepheids. The properties of the remaining stars in these two groups are discussed but their nature remains uncertain. Unexplained differences exist between the sample of stars studied here and a previous sample drawn from the NSVS by Akerlof et al. This suggests serious biases in the identification of variables in different surveys.

3 machine-readable text tables are attached as supplemental files.

PHOTOMETRY OF TYPE II CEPHEID CANDIDATES FROM THE ROTSE-I DEMONSTRATION PROJECT

Edward G. Schmidt et al. — Mon, 20 Aug 2007 09:35:04 PDT

We have obtained VR photometry of 205 Cepheid variable star candidates that were discovered in a small section of the Northern Sky Variability Survey. Given their locations and apparent magnitudes, any stars in this sample that are Cepheids are very likely to be type II Cepheids. On the basis of the regularity of variation, revised periods, light-curve morphology, slope of the color-magnitude relation, and color, we have identified 97 probable Cepheids and 17 possible Cepheids. While some of the photometric properties, particularly the colors, are consistent with the identification of these stars as Cepheids, the amplitudes are small compared to known type II Cepheids and the period distribution does not match that of known type II Cepheids.

Three supplemental data tables are attached as supplemental files.

YELLOW GIANTS IN YOUNG CLUSTERS. I. PHOTOMETRIC OBSERVATIONS

Edward G. Schmidt — Tue, 31 Oct 2006 10:25:33 PST

To enable a study of the properties of luminous yellow stars, a list has been compiled of 84 stars which appear to be in the appropriate region in the color-magnitude diagrams of 41 young clusters. Proper motions, radial velocities, UBV photometry, and spectral types from the literature have been used in those cases where such data are available to assign membership probabilities. DDO and uvby photometry have also been obtained of most of the stars in the list, and these data are analyzed to remove stars from the sample which have the wrong luminosity or the wrong color excess for cluster membership. In addition, VRI photometry is given for many of the stars in the list. Based on these data, 22 of the stars (26%) from the original sample are likely cluster members, whole 48 (57%) are not likely to be members. The remainder must be studied further to determine their status but it is probable that some of them are also in clusters.

YELLOW GIANTS IN YOUNG CLUSTERS. II. A COMPARISON OF OBSERVATION WITH THEORY

Edward G. Schmidt — Tue, 31 Oct 2006 10:19:48 PST

Luminosities and effective temperatures are calculated for 17 nonvariable yellow giants and supergiants and nine Cepheids which were previously identified as members of open clusters. These stars are identified as being in the helium core burning stage of evolution. A comparison with the predictions of pulsation theory does not show any disagreement. On the other hand, several comparisons with the theory of stellar evolution produce discrepancies which suggest that effects neglected in the calculations may play an important role in the evolution of these stars.