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The Nainital-Cape Survey. II. Report for pulsation in five chemically peculiar A-type stars and presentation of 140 null results
Aims.We search for photometric variability in chemically peculiar A typestars in the northern hemisphere. Methods: .High-speed photometricobservations of Ap and Am star candidates have been carried out fromARIES (Manora Peak, Nainital) using a three-channel fast photometerattached to the ARIES 104-cm Sampurnanand telescope. Results:.This paper presents three new variables: HD 113878, HD 118660 and HD207561. During the time span of the survey (1999 December to 2004January) pulsations of the δ Sct type were also found for the twoevolved Am stars HD 102480 and HD 98851, as reported in Joshi et al.(2002, 2003). Additionally, we present 140 null results of the surveyfor this time span. Conclusions: .The star HD 113878 pulsates witha period of 2.31 h, which is typical of δ Sct stars. HD 118660exhibits multi-periodic variability with a prominent period of nearly 1h. These periods need to be investigated and make HD 118660 aparticularly interesting target for further observations. For HD 207561,a star classified as Am, a probable pulsation with a period of 6 min wasfound in the light curves obtained on two consecutive nights. Both HD102480 and HD 98851 exhibit unusual alternating high and low amplitudemaxima, with a period ratio of 2:1. The analysis of the null resultsconfirms the photometric quality of the Nainital site.
| A statistical analysis of the detection limits of fast photometry
This work investigates the statistical limits for the detection ofstellar variability using ground based fast photometry. We show thatwhen sky transparency variations are very low or have been efficientlyremoved from the raw light curve, the overall noise is of a MixedPoisson (MP) nature (photon noise mixed by scintillation). As aconsequence, three regimes appear for the detection of photometricvariations depending on the star's brightness (scintillation,scintillation and photon noise, photon noise and sky background). Theproposed analysis is mainly applied to the Indian sites of Manora Peak(existing 104 cm telescope) and Devasthal (future 1 m automatedtelescope, and 3 m telescope project). As shown by some examples, it canbe applied to any site with the corresponding parameters. For 1 m classtelescopes at an altitude of about 2000 m, the frontier magnitudesbetween the different detection regimes are about 10 mag and 15 mag. Byanalysing the corresponding statistics of the MP noise periodogram, theminimum amplitude variation that one can detect with a given confidencelevel is evaluated for each observational setting. For example, with a 3m telescope at about 2500 m, ≈120 μmag variations would bedetected in 2 h with a 99% confidence level for stars brighter thanmagnitude 12. For a star of 15th magnitude, ≈400 μmag oscillationswould still be detected at that level. These detection limits arediscussed in the light of observations obtained in Manora peak, andcompared to results obtained at different astronomical sites.
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