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Register a new userLight Curves and Period Changes of Type II Cepheids in the Globular Clusters M3 and M5
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Light curves in the B, V, and I_c passbands have been obtained for the type II Cepheids V154 in M3 and V42 and V84 in M5. Alternating cycle behavior, similar to that seen among RV Tauri variables, is confirmed for V84. Old and new observations, spanning more than a century, show that V154 has increased in period while V42 has decreased in period. V84, on the other hand, has shown large, erratic changes in period that do not appear to reflect the long term evolution of V84 through the HR diagram.
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Mira variables, RR Lyrae variables, and type II Cepheids all represent evolved states of low-mass stars, and long term observations have revealed that changes in pulsation period occur for each of these classes of variable. Most Mira variables show small or no period changes, but a few show large period changes that can plausibly be associated with thermal pulses on the asymptotic red giant branch. Individual RR Lyrae stars show period changes that do not accord with the predictions of stellar evolution theory. This may be especially true for RR Lyrae stars that exhibit the Blazhko effect. However, the average period changes of all of the RR Lyrae variables within a globular cluster prove a better but still imperfect match for the predictions of evolutionary theory. The observed period changes of short period type II Cepheids (BL Her stars) as well as those of long period type II Cepheids (W Vir stars) are in broad agreement with the rates of period changes expected from their evolutionary motions through the instability strip.
This paper analyzes the first secured four color light curves of V396 Mon using the 2003 version of the WD code. It is confirmed that V396 Mon is a shallow W-type contact binary system with a mass ratio $q=2.554(pm0.004)$ and a degree of contact factor $f=18.9%(pm1.2%)$. A period investigation based on all available data shows that the period of the system includes a long-term decrease ($dP/dt=-8.57times{10^{-8}}$ days/year) and an oscillation ($A_3=0.^{d}0160$; $T_3=42.4,years$). They are caused by angular momentum loss (AML) and light-time effect, respectively. The suspect third body perhaps is a small M-type star (about 0.31 solar mass). Though some proofs show that this system has strong magnetic activity, through analyzing we found that the Applegate mechanism cannot explain the periodic changes. This binary is an especially important system according to Qians statistics of contact binaries as its mass ratio lies near the proposed pivot point about which the physical structure of contact binaries supposedly oscillate.
We present a compilation of UBV RIz light curves of 51 type II supernovae discovered during the course of four different surveys during 1986 to 2003: the Cerro Tololo Supernova Survey, the Calan/Tololo Supernova Program (C&T), the Supernova Optical and Infrared Survey (SOIRS), and the Carnegie Type II Supernova Survey (CATS). The photometry is based on template-subtracted images to eliminate any potential host galaxy light contamination, and calibrated from foreground stars. This work presents these photometric data, studies the color evolution using different bands, and explores the relation between the magnitude at maximum brightness and the brightness decline parameter (s) from maximum light through the end of the recombination phase. This parameter is found to be shallower for redder bands and appears to have the best correlation in the B band. In addition, it also correlates with the plateau duration, being thus shorter (longer) for larger (smaller) s values.
We discuss the observed pulsation properties of Type II Cepheids (TIICs) in the Galaxy and Magellanic Clouds. The period (P) distributions, luminosity amplitudes and population ratios of the three different sub-groups (BL Herculis[BLH, P<5 days], W Virginis [WV, 5<P<20 days], RV Tauri [RVT, P>20 days]) are quite similar in different stellar systems, suggesting a common evolutionary channel and a mild dependence on both metallicity and environment. We present a homogeneous theoretical framework based on Horizontal Branch (HB) evolutionary models, envisaging that TIICs are mainly old (t<10 Gyr), low-mass stars. The BLHs are predicted to be post early asymptotic giant branch (PEAGB) stars (double shell burning) on the verge of reaching their AGB track (first crossing of the instability strip), while WVs are a mix of PEAGB and post-AGB stars (hydrogen shell burning) moving from cool to hot (second crossing). Thus suggesting that they are a single group of variable stars. RVTs are predicted to be a mix of post-AGB stars along their second crossing (short-period tail) and thermally pulsing AGB stars (long-period tail) evolving towards their white dwarf cooling sequence. We also present several sets of synthetic HB models by assuming a bimodal mass distribution along the HB. Theory suggests, in agreement with observations, that TIIC pulsation properties marginally depend on metallicity. Predicted period distributions and population ratios for BLHs agree quite well with observations, while those for WVs and RVTs are almost a factor of two smaller and larger than observed, respectively. Moreover, the predicted period distributions for WVs peak at periods shorter than observed, while those for RVTs display a long period tail not supported by observations. We investigate several avenues to explain these differences, but more detailed calculations are required to address them.
Recent work has revealed two classes of Globular Clusters (GCs), dubbed Type-I and Type-II. Type-II GCs are characterized by a blue- and a red- red giant branch composed of stars with different metallicities, often coupled with distinct abundances in the slow-neutron capture elements (s-elements). Here we continue the chemical tagging of Type-II GCs by adding the two least-massive clusters of this class, NGC1261 and NGC6934. Based on both spectroscopy and photometry, we find that red stars in NGC1261 are slightly enhanced in [Fe/H] by ~0.1 dex and confirm that red stars of NGC 6934 are enhanced in iron by ~0.2 dex. Neither NGC1261 nor NGC6934 show internal variations in the s-elements, which suggests a GC mass threshold for the occurrence of s-process enrichment. We found a significant correlation between the additional Fe locked in the red stars of Type-II GCs and the present-day mass of the cluster. Nevertheless, most Type II GCs retained a small fraction of Fe produced by SNe II, lower than the 2%; NGC6273, M54 and omega Centauri are remarkable exceptions. In the appendix, we infer for the first time chemical abundances of Lanthanum, assumed as representative of the s-elements, in M54, the GC located in the nucleus of the Sagittarius dwarf galaxy. Red-sequence stars are marginally enhanced in [La/Fe] by 0.10pm0.06 dex, in contrast with the large [La/Fe] spread of most Type II GCs. We suggest that different processes are responsible for the enrichment in iron and s-elements in Type-II GCs.
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