arXiv subjects
Publications and source records attributed to L. L. Kiss.
We review the recent impact of microlensing projects on our understanding of pulsating red giant stars. Discussed are red giant stars' pulsation properties (period--luminosity relations, period changes, mode switchings), Red Giant Branch pulsations, metallicity effects and the use of red giant variables to explore galactic structure.
We present our first results for a sample of southern high-amplitude delta Scuti stars (HADS), based on a spectrophotometric survey started in 2003. For CY Aqr and AD CMi, we found very stable light and radial velocity curves; we confirmed the double-mode nature of ZZ Mic, BQ Ind and RY Lep. Finally, we detected gamma-velocity changes in RS Gru and RY Lep
We present the first results of our variability survey for the globular cluster NGC 362. We found numerous variable stars in the field including 23 RR Lyr stars, 4 eclipsing binaries and 5 other pulsating stars. Four of the RR Lyr ae stars are located behind the cluster, possibly in the farthest extensions of the SMC.
We present spectroscopic observations of the black-hole binary V4641 Sagittarii, obtained between 4th July 2004 and 28th March 2005, which cover the minor outburst of the star in early July 2004 and quiescence variations on 19 nights scattered over six months. During the outburst, the star peaked approximately 3 magnitudes brighter than usual, and our spectra were dominated by broad hydrogen, helium and iron emission lines. The very first spectra showed P Cygni profiles, which disappeared within a few hours, indicating rapid changes in matter ejection. The H-alpha line had multiple components, one being a broad blue-shifted wing exceeding 5000 km/s. During a simultaneously observed 10-min photometric flare-up, the equivalent width of the H-alpha line temporarily decreased, implying that it was a flare of the continuum. The overall spectral appearance was similar to that observed in the 1999 September active phase, which suggests that similar mass-ejection processes were associated with both eruptions. In quiescence, the spectra were those of the early-type secondary star showing its orbital motion around the primary. By measuring cross-correlation radial velocities, we give an improved set of spectroscopic elements. Whereas we measure the same velocity amplitude (K_2=211.3+/-1.0 km/s), within errors, as Orosz et al. (2001), our centre-of-mass velocity (v_gamma=72.7+/-3.3 km/s) differs significantly from the previously published value (107.4+/-m2.9 km/s). However, we find evidence that the difference is caused by a systematic error in data reduction in the previous study, rather than by gravitational effects of an invisible third component.
We analyse visual observations of the pulsations of the red giant variable L2 Pup. The data cover 77 years between 1927 and 2005, thus providing an extensive empirical base for characterizing properties of the oscillations. The power spectrum of the light curve shows a single mode resolved into multiple peaks under a narrow envelope. We argue that this results from stochastic excitation, as seen in solar oscillations. The random fluctuations in phase also support this idea. A comparison with X Cam, a true Mira star with the same pulsation period, and W Cyg, a true semiregular star, illustrates the basic differences in phase behaviours. The Mira shows very stable phase, consistent with excitation by the kappa-mechanism, whereas W Cyg shows large phase fluctuations that imply stochastic excitation. We find L2 Pup to be intermediate, implying that both mechanisms play a role in its pulsation. Finally, we also checked the presence of low-dimensional chaos and could safely exclude it.
We present CCD photometric observations of 23 main-belt asteroids, of which 8 have never been observed before thus the data of these objects are the first in the literature. The majority showed well detectable light variations, exceeding 0.1 mag. We have determined synodic periods for 756 Lilliana (9.36h), 1270 Datura (3.4h), 1400 Tirela (13.36h), 1503 Kuopio (9.98h), 3682 Welther (3.59h), 7505 Furushu (4.14h) and 11436 1969 QR (2.6h), while uncertain period estimates were possible for 469 Argentina (12.3h), 546 Herodias (10.4h) and 1026 Ingrid (5.3h). The shape of the lightcurves of 3682 Welther changed on a short time-scale and showed dimmings that might be attributed to eclipses in a binary system. For the remaining objects, only lower limits of the periods and amplitudes were concluded.
We present an analysis of the 3-D structure of the Magellanic Clouds, using period-luminosity (P-L) relations of pulsating red giants in the OGLE-II sample. By interpreting deviations from the mean P-L relations as distance modulus variations, we examine the three-dimensional distributions of the sample. The results for the Large Magellanic Cloud, based solely on stars below the tip of the Red Giant Branch, confirm previous results on the inclined and possibly warped bar of the LMC. The depth variation across the OGLE-II field is about 2.4 kpc, interpreted as the distance range of a thin but inclined structure. The inclination angle is about 29 deg. A comparison with OGLE-II red clump distances revealed intriguing differences that seem to be connected to the red clump reddening correction. A spatially variable red clump population in the LMC can explain the deviations, which may have a broader impact on our understanding of the LMC formation history. For the Small Magellanic Cloud, we find a complex structure showing patchy distribution scattered within 3.2 kpc of the mean. However, the larger range of the overall depth on every line-of-sight is likely to smooth out significantly the real variations.
We investigate blending, binarity and photometric biases in crowded-field CCD imaging. For this, we consider random blend losses, which correspond to the total number of stars left undetected in unresolved blends. We present a simple formula to estimate blend losses, which can be converted to apparent magnitude biases using the luminosity function of the analyzed sample. Because of the used assumptions, our results give lower limits of the total bias and we show that in some cases even these limits point toward significant limitations in measuring apparent brightnesses of ``standard candle'' stars, thus distances to nearby galaxies. A special application is presented for the OGLE-II BVI maps of the Large Magellanic Cloud. We find a previously neglected systematic bias up to 0.2-0.3 mag for faint stars (V~18.0-19.0) in the OGLE-II sample, which affects LMC distance measurements using RR Lyrae and red clump stars. We also consider the effects of intrinsic stellar correlations, i.e. binarity, via calculating two-point correlation functions for stellar fields around seven recently exploded classical novae. In two cases, for V1494 Aql and V705 Cas, the reported close optical companions seem to be physically correlated with the cataclysmic systems. Finally, we find significant blend frequencies up to 50-60% in the samples of wide-field exoplanetary surveys, which suggests that blending calculations are highly advisable to be included into the regular reduction procedure.
New optical photometric and spectroscopic data of the bright hypernova SN 2002ap are presented. The obtained BVRI light curves as well as the optical spectra agree well with other published data series. The distance has been inferred by applying the Expanding Photosphere Method for the data around maximum. The derived 6.7 Mpc is in good agreement with recent photometric distances of M74. However, the total (random plus systematic) uncertainty of the EPM-distance is at least 4.5 Mpc (about 70 %). The physical parameters of the SN have been determined via simplified analytic models of the light and velocity curves. It is confirmed that SN 2002ap was a less energetic hypernova, the kinetic energy was 4 - 8 10^51 erg, and the reddening-free absolute bolometric magnitude reached -16.63 mag (corresponding to L_bol = 3.47 10^8 L_solar), about 2 magnitude less than the prototype hypernova SN 1998bw.
We present a spectroscopic monitoring of the transient nova V4745 Sagittarii (Nova Sgr 2003 #1) covering ten months after the discovery. During this period the light curve showed well expressed transient phase in the form of six rebrightenings, and the presented dataset is one of the best spectroscopic coverages of a classical nova during the transition phase. Low- and medium-resolution spectra revealed that the majority of spectral lines switched back to strong P-Cyg profiles during the mini-outbursts, similar to the spectra just after the major eruption. We interpret the observed phenomena as evidence for ``echo-outbursts'' resulting in episodes of secondary mass-ejections and propose that the transition phase in classical novae is driven by repetitive instabilities of the hydrogen shell burning on the surface of the white dwarf. From the available light curve data we estimate the absolute magnitude of the system of about -8.3 +/- 0.5 mag. All spectrophotometric pieces of evidence point toward a very low (E(B-V) < 0.1) interstellar reddening, leading to a rough distance estimate of V4745 Sgr (9 kpc < d < 19 kpc).
A detailed light curve analysis is presented for a first-overtone RR Lyrae star, MACHO* J050918.712-695015.31, based on MACHO and OGLE-III observations. As a foreground object of the Large Magellanic Cloud, it gives an extraordinary opportunity to study an almost continuous, 12-year long dataset of a relatively bright (V~15.0 mag) RRc star with rapid period change. Cyclic period modulation is suggested by the O-C method, where the cycle length is about 8 years. With the available unique dataset, we could draw strong limits on other light curve changes that may be associated to the period modulation. We could exclude both multiple periodicity and amplitude modulation unambiguously. Any theoretical model should reproduce the observed lack of photometric modulations. Simple arguments are also given for possible hydromagnetic effects.
New observations of the eclipsing nova V1494 Aql were analysed in order to estimate the effect of the optical companion located 1.4 arcsec southwest. R and I-band images (stellar FWHM~1.2-1.4 arcsec) taken close to the primary minimum were used to determine relative magnitude differences between the nova and the companion, which were found to be Delta R~0.20 mag and Delta I~0.52 mag (in the sense nova minus companion). After correcting R-band time-series observations for the secondary light, the eclipse depth has been found to be almost twice as deep. Before modelling any late eclipsing light curve that is available in the literature, the data must be corrected for the light of the companion.
We present period-luminosity relations for more than 3,200 red variable stars in the Small Magellanic Cloud observed in the second phase of the Optical Gravitational Lensing Experiment (OGLE-II). Periods of multiply-periodic light curve solutions combined with the single-epoch 2MASS JHK_S magnitudes, reveal very similar distributions to those for the Large Magellanic Cloud in Paper I. The main features include four pulsating Asymptotic Giant Branch (AGB) ridges, three distinct short-period sequences below the tip of the Red Giant Branch and two long-period sequences of ambiguous origin. We derive a relative distance modulus of the Clouds from the period-luminosity distributions for all stars of Delta mu_0=0.44 mag, which is in good agreement with recent independent results. The tip of the Red Giant Branch shows a colour and metallicity dependence that is in excellent agreement with the empirical results for globular clusters. We conclude that most variable stars below the TRGB are indeed RGB stars.
We report a detection of statistically significant color variations for a sample of 7,531 multiply observed asteroids that are listed in the Sloan Digital Sky Survey Moving Object Catalog. Using 5-band photometric observations accurate to ~0.02 mag, we detect color variations in the range 0.06-0.11 mag (rms). These variations appear uncorrelated with asteroids physical characteristics such as diameter (in the probed 1-10 km range), taxonomic class, and family membership. Despite such a lack of correlation, which implies a random nature for the cause of color variability, a suite of tests suggest that the detected variations are not instrumental effects. In particular, the observed color variations are incompatible with photometric errors, and, for objects observed at least four times, the color change in the first pair of observations is correlated with the color change in the second pair. These facts strongly suggest that the observed effect is real, and also indicate that for some asteroids color variations are larger than for others. The detected color variations can be explained as due to inhomogeneous albedo distribution over an asteroid surface. Although relatively small, these variations suggest that fairly large patches with different color than their surroundings exist on a significant fraction of asteroids. This conclusion is in agreement with spatially resolved color images of several large asteroids obtained by NEAR spacecraft and HST.
We present the first CCD photometric observations of the northern open cluster NGC 2126. Data were taken on eight nights in February and December 2002 with a total time span of ~57 hours. Almost 1000 individual V-band frames were examined to find short-period variable stars. We discovered six new variable stars, of which one is a promising candidate for an eclipsing binary with a pulsating component. Two stars were classified as delta Scuti stars and one as Algol-type eclipsing binary. Two stars are slow variables with ambiguous classification. From absolute VRI photometry we have estimated the main characteristics of the cluster: m-M=11.0+/-0.5, E(V-I)=0.4+/-0.1, E(V-R)=0.08+/-0.06 (E(B-V)=0.2+/-0.15) and d=1.3+/-0.6 kpc. Cluster membership is suggested for three variable stars from their positions on the colour-magnitude diagram.
Observations of the red giant Arcturus (Alpha Boo) obtained with the star tracker on the Wide Field Infrared Explorer (WIRE) satellite during a baseline of 19 successive days in 2000 July-August are analysed. The amplitude spectrum has a significant excess of power at low-frequencies. The highest peak is at about 4.1 micro-Hz (2.8 d), which is in agreement with previous ground-based radial velocity studies. The variability of Arcturus can be explained by sound waves, but it is not clear whether these are coherent p-mode oscillations or a single mode with a short life-time.
We present period-luminosity relations for more than 23,000 red giants in the Large Magellanic Cloud observed by the OGLE-II microlensing project. The OGLE period values were combined with the 2MASS single-epoch JHK photometric data. For the brighter stars we find agreement with previous results (four different sequences corresponding to different modes of pulsation in AGB stars). We also discovered two distinct and well-separated sequences below the tip of the Red Giant Branch. They consist of almost 10,000 short-period (15 d <P< 50 d), low-amplitude (A_I<0.04 mag) red variable stars, for which we propose that a significant fraction is likely to be on the Red Giant Branch, showing radial pulsations in the second and third overtone modes. The excitation mechanism could be either Mira-like pulsation or solar-like oscillations driven by convection.
We present new photometric data for seven high-amplitude delta Scuti stars. The observations were acquired between 1996 and 2002, mostly in the Johnson photometric system. For one star (GW UMa), our observations are the first since the discovery of its pulsational nature from the Hipparcos data.The primary goal of this project was to update our knowledge on the period variations of the target stars. For this, we have collected all available photometric observations from the literature and constructed decades-long O-C diagrams of the stars. This traditional method is useful because of the single-periodic nature of the light variations. Text-book examples of slow period evolution (XX Cyg, DY Her, DY Peg) and cyclic period changes due to light-time effect (LITE) in a binary system (SZ Lyn) are updated with the new observations. For YZ Boo, we find a period decrease instead of increase. The previously suggested LITE-solution of BE Lyn (Kiss & Szatmary 1995) is not supported with the new O-C diagram. Instead of that, we suspect the presence of transient light curve shape variations mimicking small period changes.