arXiv subjects

S. Frandsen

Publications and source records attributed to S. Frandsen.

65 records · Page 4Linked to original sources

arXiv2003

A Search for Planets in the Old Open Cluster NGC 6791

We describe the results of a search for transit-like events caused by giant planets occulting stars in the old, metal-rich open cluster NGC 6791 based on BVI-colour photometry from eight nights of observations with the 2.54m Nordic Optical Telescope. To extract the light curves we have used both PSF photometry (DAOPHOT) and the difference imaging technique (ISIS). We have re-analyzed observations from earlier campaigns to search for multiple transits, determine periods of long-period variables, and detect eclipsing binaries. We confirm 20 known variables and have discovered 22 new low-amplitude variables with amplitudes in the range 7--40 mmag and periods 1--16 days. We have found the primary and secondary eclipses of two eclipsing binaries based on the new and older photometric campaigns. The search for transits-like events has turned up a few single-transit candidates. The transit depths are 10 mmag in both the V and I filters over periods of 1.0--2.5 hours, but future observations are required to see whether identical transit-like events recur in these same stars.

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arXiv2003

Confirmation of solar-like oscillations in eta Bootis

We obtained time-series spectroscopy of the G0 subgiant eta Boo in an attempt to confirm the solar-like oscillations reported by Kjeldsen et al. (1995). We recorded 1843 spectra over six consecutive nights with the Nordic Optical Telescope, which we used to measure equivalent widths of strong temperature-sensitive lines. We also measured velocities from 1989 spectra obtained through an iodine reference cell at Lick Observatory over 56 nights that were badly affected by weather. Our analysis also included velocity measurements published by Brown et al. (1997) and the original equivalent-width measurements by Kjeldsen et al. (1995). All four data sets show power excesses consistent with oscillations, although with a range of amplitudes that presumably reflects the stochastic nature of the excitation. The highest peaks show regularity with a large separation of Delta nu = 40.4 microHz and we identify 21 oscillation frequencies from the combined data.

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arXiv2002

Detection of Solar-like Oscillations in the G7 Giant Star xi Hya

We report the firm discovery of solar-like oscillations in a giant star. We monitored the star xi Hya (G7III) continuously during one month with the CORALIE spectrograph attached to the 1.2m Swiss Euler telescope. The 433 high-precision radial-velocity measurements clearly reveal multiple oscillation frequencies in the range 50 - 130 uHz, corresponding to periods between 2.0 and 5.5 hours. The amplitudes of the strongest modes are slightly smaller than 2 m/s. Current model calculations are compatible with the detected modes.

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arXiv2002

Rotation of stars in NGC 6134: a comparison of Delta Scuti stars and non-variable stars

We present results of spectroscopic observations of selected stars in the southern open cluster NGC 6134. We have determined the rotational velocities of the six known Delta Scuti Stars in NGC 6134 as well as several other non-variable stars with similar colour temperature in order to investigate if v sin i and variability is somehow connected: we find no such correlation. We also compare the distribution of v sin i of Delta Scuti Stars and non-variable stars with four other well-studied open clusters to look for any systematic behaviour, but we find no conclusive evidence for v sin i and variability to be connected. We have also used the spectra to carry out an abundance analysis of the Delta Scuti Stars in NGC 6134 to confirm the high metal content of the cluster. We find [Fe/H] = +0.38 +/- 0.05 which is in agreement with the result obtained from Stromgren photometry. We also present Delta-a photometry of the cluster, but we find no chemical peculiar stars based on this index.

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arXiv2002

Confirmation of Solar-Like Oscillations in eta Bootis

We report observations of the G0 subgiant eta Boo made in 1998, in Balmer-line equivalent width with the 2.5-m Nordic Optical Telescope and in velocity with the 24-inch Lick CAT. In both data sets we see an excess in the power spectrum, with oscillation frequencies that confirm the earlier observations by Kjeldsen et al. (1995).

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arXiv2001

SX Phoenicis Stars in the Core of 47 Tucanae

We present new results on five of six known SX Phoenicis stars in the core of the globular cluster 47 Tucanae. We give interpretations of the light curves in the V and I bands from 8.3 days of observations with the Hubble Space Telescope near the core of 47 Tuc. The most evolved SX Phe star in the cluster is a double-mode pulsator (V2) and we determine its mass to be 1.54+/-0.05 solar masses from its position in the Hertzsprung-Russell diagram and by comparing observed periods with current theoretical pulsation models. For V14 we do not detect any pulsation signal. For the double-mode pulsators V3, V15, and V16 we cannot give a safe identification of the modes. We also describe the photometric techniques we have used to extract the light curves of stars in the crowded core. Some of the SX Phoenicis are saturated and we demonstrate that even for stars that show signs of a bleeding signal we can obtain a point-to-point accuracy of 1-3%.

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arXiv2000

A Lack of Planets in 47 Tucanae from an HST Search

We report results from a large Hubble Space Telescope project to observe a significant (~34,000) ensemble of main sequence stars in the globular cluster 47 Tucanae with a goal of defining the frequency of inner-orbit, gas-giant planets. Simulations based on the characteristics of the 8.3 days of time-series data in the F555W and F814W WFPC2 filters show that ~17 planets should be detected by photometric transit signals if the frequency of hot Jupiters found in the solar neighborhood is assumed to hold for 47 Tuc. The experiment provided high-quality data sufficient to detect planets. A full analysis of these WFPC2 data reveals ~75 variables, but NO light curves resulted for which a convincing interpretation as a planet could be made. The planet frequency in 47 Tuc is at least an order of magnitude below that for the solar neighborhood. The cause of the absence of close-in planets in 47 Tuc is not yet known; presumably the low metallicity and/or crowding of 47 Tuc interfered with planet formation, with orbital evolution to close-in positions, or with planet survival.

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arXiv1998

A search for solar-like oscillations and granulation in alpha Cen A

We report the most sensitive search yet made for solar-like oscillations. We observed the star alpha Cen A in Balmer-line equivalent widths over six nights with both the 3.9m Anglo-Australian Telescope and the ESO 3.6m telescope. We set an upper limit on oscillations of 1.4 times solar and found tentative evidence for p-mode oscillations. We also found a power excess at low frequencies which has the same slope and strength as the power seen from granulation in the Sun. We therefore suggest that we have made the first detection of temporal fluctuations due to granulation in a solar-like star.

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arXiv1998

Spectroscopy of the roAp star Alpha-Cir -- II. The bisector and equivalent-width of the H-alpha line

We present bisector measurements of the H-alpha line of the rapidly oscillating Ap (roAp) star, Alpha-Cir, obtained from dual-site observations with medium-dispersion spectrographs. The velocity amplitude and phase of the principal pulsation mode vary significantly, depending on the height in the H-alpha line, including a phase reversal between the core and the wings of the line. This supports the theory, suggested in Paper I, of a radial pulsational node in the atmosphere of the star. Blending with metal lines partially affects the H-alpha bisector results but probably not enough to explain the phase reversal. We have also detected changes in the equivalent-width of the line during the pulsation, and measured the oscillatory signal as a function of wavelength across the H-alpha region.

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arXiv1998

Oscillation mode identifications and models for the delta Scuti star FG Virginis

We present new spectroscopic and photometric time series observations of the delta Scuti star FG~Vir. We detect the oscillations via changes in the equivalent widths of hydrogen and metal absorption lines. From the ratios between spectroscopic and photometric amplitudes, we assign l values to the eight strongest oscillation modes. In particular, we identify two radial modes (l =0) and find that the main pulsation mode (147 microHz) has l = 1. One of the radial modes (at 140 microHz) is the fundamental, implying that two modes with lower frequencies are g-modes. For the radial modes, we compare frequencies with those calculated from a scaled delta Scuti star model and derive a density 0.1645 +/- 0.0005 rho_sol. We then obtain a distance of 84 +/- 3 pc, in excellent agreement with the Hipparcos value. Finally, we suggest that a 3.5-day variability in all observables (equivalent widths and intensity) is caused by stellar rotation.

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arXiv1997

A search for solar-like oscillations in alpha Cen A

We have been using a new method to search for solar-like oscillations that involves measuring temperature changes via their effect on the equivalent widths of the Balmer hydrogen lines. We observed alpha Cen A over six nights in 1995 with the 3.9-metre Anglo-Australian Telescope and the European Southern Observatory's 3.6-metre telescope in Chile. Oscillations were not detected, with an upper limit only slightly higher than the expected signal.

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