Search for evolutionary changes in Cepheid periods using the Harvard plate collection: NSV 9159

We have obtained 530 photographic magnitude estimates for the long-period classical Cepheid NSV 9159 (P = 39^d) in the plate collections of the Harvard Observatory and the Sternberg Astronomical Institute. Together with the currently available CCD observations from the ASAS-3 catalog, our data have allowed us to construct an O-C diagram spanning a time interval of 119 years. The O-C diagram has the shape of a parabola, which has made it possible to determine for the first time the quadratic light elements and to calculate the rate of evolutionary decrease in the period, 314.4 (±7.3) s yr^?1, in agreement with the results of theoretical calculations for the second crossing of the instability strip. The available data reduced by the Eddington-Plakidis method do not reveal any noticeable random fluctuations in the period.     

Search for evolutionary changes in Cepheid periods using the Harvard plate collection: ASAS 101538-5933.1

In the plate collection of the Harvard College Observatory, we have obtained 528 photographic magnitude estimates for the recently discovered long-period classical Cepheid ASAS 101538-5933.1 (P = 51.4 days). Together with the published photoelectric and CCD observations, our data have allowed us to construct an O-C diagram spanning a time interval of 120 years. The O-C diagram has the shape of a parabola, which has made it possible to determine for the first time the quadratic light elements and to calculate the rate of evolutionary increase in the period, dP/dt = 51.8 (±4.8) s yr⁻¹ or $ \dot P $ \dot P /P = 7.3 (±0.7) s, in agreement with the results of theoretical calculations for the third crossing of the instability strip. The available data reduced by the method of Eddington and Plakidis reveal small random period fluctuations that do not distort the evolutionary trend in the O-C residuals.     

Search for evolutionary changes in the periods of Cepheids: U Sgr

For the classical Cepheid U Sgr, we have constructed an O - C diagram spanning a time interval of 144 years. The O - C diagram has the shape of a parabola, which has made it possible to determine for the first time the quadratic light elements and to calculate the rate of evolutionary increase in the period, dP/dt = 0.39 (±0.10) s yr^?1, in agreement with the results of theoretical calculations for the third crossing of the instability strip. The available data reduced by the Eddington-Plakidis method reveal small random period fluctuations that do not distort the evolutionary trend in the O - C residuals.     

Frequency analysis of Cepheids in the Large Magellanic Cloud: new types of classical Cepheid pulsators

We have performed a detailed systematic search for multiperiodicity in the Population I Cepheids of the Large Magellanic Cloud. In this process, we have identified for the first time several new types of Cepheid pulsational behaviour. We have found two triple-mode Cepheids pulsating simultaneously in the first three radial overtones. In 9 per cent of the first overtone (FO) Cepheids, we have detected weak but well-resolved secondary periodicities. They appear either very close to the primary pulsation frequency or at a much higher frequency with a characteristic period ratio of 0.60–0.64. In either case, the secondary periodicities must correspond to non-radial modes of oscillation. This result presents a major challenge to the theory of stellar pulsations, which predicts that such modes should not be excited in Cepheid variables. Non-radial modes have also been found in three of the fundamental first overtone (FU/FO) double-mode Cepheids, but no such oscillations have been detected in single-mode Cepheids pulsating in the FU mode.
In 19 per cent of double-mode Cepheids pulsating in the first two radial overtones (FO/SO type), we have detected a Blazhko-type periodic modulation of amplitudes and phases. Both modes are modulated with a common period, which is always longer than 700 d. Variations of the two amplitudes are anticorrelated, and maximum of one amplitude always coincides with minimum of the other. We have compared observations of modulated FO/SO Cepheids with predictions of theoretical models of the Blazhko effect, showing that the currently most popular models cannot account for properties of these stars. We propose that the Blazhko effect in FO/SO Cepheids can be explained by a non-stationary resonant interaction of one of the radial modes with another, perhaps non-radial, mode of oscillations.     

Chromospheric activity of late-type giants and supergiants: Reappearance of dynamo activity in the interior due to the spin-up of the core in evolution

The renewal of chromospheric activity in red giants and supergiants is interpreted in terms of the reappearance of dynamo activity in the interior due to the spin-up of the core caused by its contraction in the course of evolution from the main sequence to the giant stage. A region of very high rotational shear (differential rotation) develops between the core, which spins up by a large factor through the drastic contraction, and the envelope, which spins down in contrast by virtue of expansion. Mechanisms of angular momentum transfer may operate to smear this large shear, and bring the inner part of the envelope into sheared rotation. A convective layer, on the other hand, develops in the envelope from the surface inwards, when the envelope expands and the temperature is lowered. A dynamo layer, or a layer in which the sheared rotation co-exists with the convection (the presence of a remnant magnetic field being postulated), will thus reappear in the inner part of the envelope when the envelope-convection reaches down and invades the layer of sheared rotation. Surface chromospheric activity due to the magnetic field is thus renewed when the regenerated magnetic field is brought up to the surface by the envelope-convection. These phenomena occur as the star evolves into the giant stage and hence explain the observed characteristic of gradual revival of chromospheric activity from the subgiant to the giant stage. Visiting Scientist at the Tokyo Astronomical Observatory under the support of the Japan Society for the Promotion of Sciences. On leave from the Indian Institute of Astrophysics. Deceased on 1982 August 19     

An analysis of the orbital periods of some eclipsing binaries suspected to be in the pre-main sequence contraction phase of evolution

An analysis of the available photoelectric times of minima of KO Aql, TV Cas and Z Her, which are suspected to be in pre-main sequence phase of evolution, reveals that KO Aql shows a secular increase in its orbital period at the rate of 4·34 × 10⁻⁸ day per cycle while the period of TV Cas has been decreasing at the rate of 4·08 × 10⁻⁹ day per cycle. Z Her does not show any period change at all. The orbital period of any binary system which is in the pre-main sequence phase will be systematically affected because of ′ shrinking′ dimensions of the components. A simple formula for the characteristic period change, defined by (P/P), is derived from a consideration of the conservation of total energy and total angular momentum for a binary system whose components are still in the process of contraction or expansion. The derived formula is applied to the above systems to see whether theoretical characteristic period changes agree with the observed values. The systems are assumed to evolve independently in the pre-main sequence phase in accordance with the model calculations of Iben (1965). It is found that there is no agreement between theoretical and observed characteristic period changes. This suggests that KO Aql and TV Cas may not be in the pre-main sequence phase. We do not have sufficient data for Z Her to judge its evolutionary status by the present procedure; this is also true of TT Hya. We suspect that the period changes observed in KO Aql and TV Cas may be due to light-time effect.     

Accurate parameter estimation for star formation history in galaxies using SDSS spectra

To further our knowledge of the complex physical process of galaxy formation, it is essential that we characterize the formation and evolution of large data bases of galaxies. The spectral synthesis starlight code of Cid Fernandes et al. was designed for this purpose. Results of starlight are highly dependent on the choice of input basis of simple stellar population (SSP) spectra. Speed of the code, which uses random walks through the parameter space, scales as the square of the number of the basis spectra, making it computationally necessary to choose a small number of SSPs that are coarsely sampled in age and metallicity. In this paper, we develop methods based on a diffusion map that, for the first time, choose appropriate bases of prototype SSP spectra from a large set of SSP spectra designed to approximate the continuous grid of age and metallicity of SSPs of which galaxies are truly composed. We show that our techniques achieve better accuracy of physical parameter estimation for simulated galaxies. Specifically, we show that our methods significantly decrease the age–metallicity degeneracy that is common in galaxy population synthesis methods. We analyse a sample of 3046 galaxies in Sloan Digital Sky Survey Data Release 6 and compare the parameter estimates obtained from different basis choices.     

Evolution of massive binary stars in the LMC and its implications for radio pulsar population

The Hertzsprung-Russell diagram of the Large Magellanic Cloud compiled recently by Fitzpatrick & Garmany (1990) shows that there are a number of supergiant stars immediately redward of the main sequence although theoretical models of massive stars with normal hydrogen abundance predict that the region 4.5 ≤ logT _eff ≤ 4.3 should be un-populated (“gap”). Supergiants having surface enrichment of helium acquired for example from a previous phase of accretion from a binary companion, however, evolve in a way so that the evolved models and observed data are consistent — an observation first made by Tuchman & Wheeler (1990). We compare the available optical data on OB supergiants with computed evolutionary tracks of massive stars of metallicity relevant to the LMC with and without helium-enriched envelopes and conclude that a large fraction (≈ 60 per cent) of supergiant stars may occur in binaries. As these less evolved binaries will later evolve into massive X-ray binaries, the observed number and orbital period distribution of the latter can constrain the evolutionary scenarios of the supergiant binaries. The distributions of post main sequence binaries and closely related systems like WR + O stars are bimodal-consisting of close and wide binaries in which the latter type is numerically dominating. When the primary star explodes as a supernova leaving behind a neutron star, the system receives a kick and in some cases can lead to runaway O-stars. We calculate the expected space velocity distribution for these systems. After the second supernova explosion, the binaries in most cases, will be disrupted leading to two runaway neutron stars. In between the two explosions, the first born neutron star’s spin evolution will be affected by accretion of mass from the companion star. We determine the steady-state spin and radio luminosity distributions of single pulsars born from the massive stars under some simple assumptions. Due to their great distance, only the brightest radio pulsars may be detected in a flux-limited survey of the LMC. A small but significant number of observable single radio pulsars arising out of the disrupted massive binaries may appear in the short spin period range. Most pulsars will have a low velocity of ejection and therefore may cluster around the OB associations in the LMC.     

Formation of galactic subsystems in light of the magnesium abundance in field stars: The thick disk

The space velocities and Galactic orbital elements of stars calculated from the currently available high-accuracy observations in our compiled catalog of spectroscopic magnesium abundances in dwarfs and subgiants in the solar neighborhood are used to identify thick-disk objects. We analyze the relations between chemical, spatial, and kinematic parameters of F–G stars in the identified subsystem. The relative magnesium abundances in thick-disk stars are shown to lie within the range 0.0 < [Mg/Fe] < 0.5 and to decrease with increasingmetallicity starting from [Fe/H] ≈ ?1.0. This is interpreted as evidence for a longer duration of the star formation process in the thick disk. We have found vertical gradients in metallicity (grad_Z[Fe/H] = ?0.13 ± 0.04 kpc^?1) and relative magnesium abundance (grad_Z[Mg/Fe] = 0.06 ± 0.02 kpc^?1), which can be present in the subsystem only in the case of its formation in a slowly collapsing protogalaxy. However, the gradients in the thick disk disappear if the stars whose orbits lie in the Galactic plane, but have high eccentricities and low azimuthal space velocities atypical of the thin-disk stars are excluded from the sample. The large spread in relative magnesium abundance (?0.3 < [Mg/Fe] < 0.5) in the stars of the metal-poor “tail” of the thick disk, which constitute ≈8% of the subsystem, can be explained in terms of their formation inside isolated interstellar clouds that interacted weakly with the matter of a single protogalactic cloud. We have found a statistically significant negative radial gradient in relative magnesium abundance in the thick disk (grad_R[Mg/Fe] = ?0.03 ± 0.01 kpc^{? 1}) instead of the expected positive gradient. The smaller perigalactic orbital radii and the higher eccentricities for magnesium-richer stars, which, among other stars, are currently located in a small volume of the Galactic space near the Sun, are assumed to be responsible for the gradient inversion. A similar, but statistically less significant inversion is also observed in the subsystem for the radial metallicity gradient.     

A Search for Variability in the HCCN to H2CO Ratio in Comet Hale-Bopp

We observed submillimeter lines of H₂CO and HCN in comet Hale-Bopp near perihelion. One of our goals was to search for short term variability. Our observations are suggestive, but not conclusive, of temporal and/or spatial changes in the coma's HCN/H₂CO abundance ratio of ∼25%. If due to spatial variability, the ratio on the sunward side of the coma is enhanced over other regions. If due to temporal variability, we find the bulk ratio in the coma changed in less than 16 hours. This revised version was published online in July 2006 with corrections to the Cover Date.     

Star formation in the LMC: Comparative CCD observations of young stellar populations in two giant molecular clouds

This work deals with a CCD imaging study at optical and near‐infrared wavelength oftwo giant molecular clouds (plus a control field) in the southern region of the Large Magellanic Cloud, one ofwhich shows multiple signs of star formation, whereas the other does not. The observational data from VLT FORS2 (R band) and NTT SOFI (Ks band) have been analyzed to derive luminosity functions and color‐magnitude diagrams. The young stellar content of these two giant molecular clouds is compared and confirmed to be different, in the sense that the apparently “starless” cloud has so far formed only low‐luminosity, low‐mass stars (fainter than m_Ks ∽ 16.5 mag, not seen by 2MASS), while the other cloud has formed both faint low‐mass and luminous high‐mass stars. The surface density excess oflow‐luminosity stars (∽2 per square arcmin) in the “starless” cloud with respect to the control field is about 20% whereas the excess is about a factor of 3 in the known star‐forming cloud. The difference may be explained theoretically by the gravo‐turbulent evolution of giant molecular clouds, one being younger and less centrally concentrated than the other (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Search for star clustering: methodology and application to the Two Micron Galactic Survey

A new approach to the study of the large-scale stellar cluster distribution in the Galaxy based on two-point correlation techniques is presented. The basic formalism for this method is outlined, and its applications are then investigated by the use of a simple model of cluster distribution in the Galaxy. This provides an estimate of the potentials of the two-point correlation function for indicating clustering in the measured star positions, which can be related to the presence of star clusters in the observed field. This technique is then applied to several areas of the Two Micron Galactic Survey catalogue, from which information is obtained on the distribution of clusters according to position in the Galaxy, as well as about age, density of stars, etc.     

Search for secular changes in the 3D profile of the synchrotron radiation around Jupiter

We present a summary of Jupiter data taken over an eighteen year span (1981-1998) by the Very Large Array at ∼21.0 cm. At this wavelength the emission is dominated by synchrotron radiation, which is roughly proportional to the product of the electron number density and magnetic field strength (N_eB). At each epoch 8-12 hours of data were taken, which allowed us to examine Jupiter during an entire rotation period. We mapped the longitudinal structure of the synchrotron radiation by using a 3D reconstruction technique developed by Sault et al. [Astron. Astrophys. 324 (1997) 1190] which enabled us to produce plots of the latitude, radial distance, and peak intensity vs. jovian longitude (System III). The results show the shape of the synchrotron radiation has remained stable (except, of course, during the period of comet Shoemaker-Levy 9 impacts). Specifically, the latitudinal structure has remained nearly constant. Furthermore, the general dependence of the radial intensity profile has remained the same throughout the years, though radial distance has slightly, though significantly, changed. This constancy implies that the spatial structure of both the particle distribution and magnetic field have varied little over the eighteen year span. The primary changes in the synchrotron radiation have been seen in the intensity of emission as a function of time. There are certain epochs (e.g., 1987) which show more emissivity than others (e.g., 1981, 1995) at all longitudes. When each epoch is longitudinally averaged, there may be an anti-correlation between the radial distance and corresponding peak intensities of the synchrotron radiation, as one might expect if radial diffusion is important. We examine these trends by comparing the data to plots of the total intensity at 13 cm (by Klein et al., in: Rucker, H.O., et al., Planetary Radio Emissions V. Austrian Acad. Sci. Press, Vienna, p. 221). Overall, variations in our 21-cm data are similar to those measured at 13 cm, but there appears to be a change in spectral index and perhaps in the spatial brightness distribution in 1992. We attribute this to a change in both the spatial and energy distribution of the relativistic electrons.