Discovery of unusual pulsations in the cool, evolved Am stars HD 98851 and HD 102480
The chemically peculiar (CP) stars HD 98851 and HD 102480 have been discovered to be unusual pulsators during the ‘Naini Tal–Cape Survey’ programme to search for pulsational variability in CP stars. Time series photometric and spectroscopic observations of these newly discovered stars are reported here. Fourier analyses of the time series photometry reveal that HD 98851 is pulsating mainly with frequencies 0.208 and 0.103 mHz, and HD 102480 is pulsating with frequencies 0.107, 0.156 and 0.198 mHz. The frequency identifications are all subject to 1 d−1 cycle count ambiguities. We have matched the observed low-resolution spectra of HD 98851 and HD 102480 in the range 3500–7400 Å with theoretical synthetic spectra using Kurucz models with solar metallicity and a micro-turbulent velocity of 2 km s−1. These yield Teff= 7000 ± 250 K, log g= 3.5 ± 0.5 for HD 98851 and Teff= 6750 ± 250 K, log g= 3.0 ± 0.5 for HD 102480. We determined the equivalent H-line spectral class of these stars to be F1 IV and F3 III/IV, respectively. A comparison of the location of HD 98851 and HD 102480 in the HR diagram with theoretical stellar evolutionary tracks indicates that both stars are about 1-Gyr-old, 2-M⊙ stars that lie towards the red edge of the δ Sct instability strip. From comparison between the observed and calculated physical parameters, we conclude that HD 98851 and HD 102480 are cool, evolved Am pulsators. The light curves of these pulsating stars have alternating high and low amplitudes, nearly harmonic (or subharmonic) period ratios, high pulsational overtones and Am spectral types. This is unusual for both Am and δ Sct pulsators, making these stars interesting objects for further observational and theoretical studies.
- [Show abstract] [Hide abstract] ABSTRACT: The discovery of a new rapidly oscillating Ap star, HD 116114, with a pulsation period of 21 min, using high-resolution spectra obtained with the Ultraviolet-Visual Echelle Spectrograph at the European Southern Observatory's Very Large Telescope, is presented. The highest amplitudes of the radial velocity variations are between 50 and 125 m s−1 visible in the Eu ii lines. The spectral lines of La ii and the core of the H line have amplitudes of about 30 m s−1. The frequency obtained for the oscillations is in good agreement with theoretical predictions of longer-period, evolved roAp stars. The distinction in luminosity between the roAp and noAp stars, and the suggestion that in all roAp stars the abundance of the second ions of Pr and Nd, relative to the abundance of the first ions, is anomalously high, need to be revised in the light of this discovery.
- [Show abstract] [Hide abstract] ABSTRACT: Wehavediscoveredthefirstexampleofastarpulsatingintrinsicallywithboth Doradusand Scutifrequencies. The star, HD 8801, is an Am metallic-line star that appears to be single. Since the vast majority of Am stars are members of binary systems with periods less than 1000 days and also do not pulsate, HD 8801 possesses a very unusual,ifnotunique,combinationofphysicalproperties.Ourphotometry,acquiredwithanautomatictelescopeat Fairborn Observatory, resolves six independent pulsation periods between 0.048 and 0.404 days. The two longest periods lie within the range of Dor variables, while the four shorter periods are within the range of Sct stars. Elevenradialvelocities,obtainedoveran11monthperiod,haveameanof0:8 0:3kms 1,providingnoevidence for a spectroscopic companion. The star is on the main sequence and lies within both the Dor and Sct instability strips. Pulsation constants computed for the six periods agree with pulsation constants computed for other Dor and Sctvariables.HD 8801is alsothe first confirmedcaseofa metallic-linestarpulsatingwith Dorfrequencies. Its projected rotational velocity of 55 km s 1 is typical for Am stars, which, as a group, rotate more slowly than normal A stars. HD 8801's slow rotation compared to normal A stars does not appear to arise from either a binary companion or evolutionary expansion and thus remains a puzzle. The coexistence of metallicism and pulsation in several main-sequence stars near the red edge of the Sct strip contradicts recent models that predict pulsations only in more evolved stars.
- [Show abstract] [Hide abstract] ABSTRACT: The “Nainital-Cape Survey” program for searching photometric variability in chemically peculiar (CP) stars was initiated in 1997 at ARIES, Nainital. We present here the results obtained to date. The Am stars HD 98851, HD 102480, HD 13079 and HD 113878 were discovered to exhibit δ Scuti type variability. Photometric variability was also discovered in HD 13038, for which the type of peculiarity and variability is not fully explained. The null results of this survey are also presented and discussed.