arXiv subjects

T. H. Dall

Publications and source records attributed to T. H. Dall.

28 records · Page 2Linked to original sources

arXiv2005

Amplitude Ratios as Mode Characterizors in delta Scuti stars

Seismology of delta Scuti stars holds great potentials for testing theories of stellar structure and evolution. The ratio of mode amplitudes in light and in equivalent width of spectral lines can be used for mode identification. However, the amplitude ratios (AR) predicted from theory are usually inconsistent with observations. We here present the first results from a campaign aimed at calibrating observationally the absolute values of the AR.

astro-ph↗

arXiv2005

Outbursts on normal stars. FH Leo misclassified as a novalike variable

We present high resolution spectroscopy of the common proper motion system FH Leo (components HD 96273 and BD+07 2411B), which has been classified as a novalike variable due to an outburst observed by Hipparcos, and we present and review the available photometry. We show from our spectra that neither star can possibly be a cataclysmic variable, instead they are perfectly normal late-F and early-G stars. We measured their radial velocities and derived the atmospheric fundamental parameters, abundances of several elements including Fe, Ni, Cr, Co, V, Sc, Ti, Ca and Mg, and we derive the age of the system. From our analysis we conclude that the stars do indeed constitute a physical binary. However, the observed outburst cannot be readily explained. We examine several explanations, including pollution with scattered light from Jupiter, binarity, microlensing, background supernovae, interaction with unseen companions and planetary engulfment. While no explanation is fully satisfactory, the scattered light and star-planet interaction scenarios emerge as the least unlikely ones, and we give suggestions for further study.

astro-ph↗

arXiv2004

First spectra of the W UMa system V524 Monocerotis

We present the first high-resolution spectra of the W UMa contact binary V524 Mon. The spectra of the two components are very similar, resembling a G5 and a K0. We find the radial velocities and rotational velocities consistent with corotation. We estimate the radii and the masses and derive a mass ratio M_2/M_1 = 2.1. We confirm that V524 Mon is a W-type contact system, likely enclosed in a common convective envelope, as found by Samec & Loflin (2003). We do not find evidence for the expected level of emission in the chromospheric CaII H and K lines, neither H_alpha, indicating that the magnetic activity is much weaker than expected or that other processes are hampering chromospheric emission.

astro-ph↗

arXiv2004

The supernova 2003lw associated with X-ray flash 031203

The X-Ray Flash (XRF), 031203 with a host galaxy at z=0.1055, is, apart from GRB980425, the closest Gamma-Ray Burst (GRB) or XRF known to date. We monitored its host galaxy from 1-100 days after the burst. In spite of the high extinction to the source and the bright host, a significant increase and subsequent decrease has been detected in the apparent brightness of the host, peaking between 10 and 33 days after the GRB. The only convincing explanation is a supernova (SN) associated with the XRF, SN2003lw. This is the earliest time at which a SN signal is clearly discernible in a GRB/XRF (apart from SN1998bw). SN2003lw is extremely luminous with a broad peak and can be approximately represented by the lightcurve of SN1998bw brightened by ~0.55 mag, implying a hypernova, as observed in most GRB-SNe. The XRF-SN association firmly links XRFs with the deaths of massive stars and further strengthens their connection with GRBs. The fact that SNe are also associated with XRFs implies that Swift may detect a significant population of intermediate redshift SNe very soon after the SN explosions, a sample ideally suited for detailed studies of early SN physics.

astro-ph↗

arXiv2002

Time-series Spectroscopy of Pulsating sdB stars III: Line Indices of PG1605+072

We present the detection and analysis of line index variations in the pulsating sdB star PG 1605+072. We have found a strong dependence of line index amplitude on Balmer line order, with high-order Balmer line amplitudes up to 10 times larger than H-beta. Using a simple model, we have found that the line index may not only be dependent on temperature, as is usually assumed for oscillating stars, but also on surface gravity. This information will provide another set of observables that can be used for mode identification of sdBs.

astro-ph↗

arXiv2002

Time-series Spectroscopy of Pulsating sdB Stars II: Velocity Analysis of PG1605+072

We present the analysis of time-resolved spectroscopy of the pulsating sdB star PG1605+072. From our main observing run of 16 nights we have detected velocity variations at 5 frequencies that correspond to those found in photometry. Based on these data, there appears to be change in amplitude of the dominant modes over about a year. However, when we include extra observations to improve the frequency resolution, we find that some of the frequencies are split into two or three. Simulations suggest that the apparent amplitude variation can be at least partially explained by a series of very closely spaced frequencies around the two strongest modes. By combining observations taken over ~300 days we conclude that some of the closely spaced modes are caused by one mode whose amplitude is varying, however this frequency is still within ~1 microHz of an apparently stable frequency. Because of this kind of complexity and uncertainty we advise caution when trying to identify oscillation modes in this star.

astro-ph↗

arXiv2001

The optical afterglow and host galaxy of GRB000926

We present the discovery of the Optical Transient (OT) of the long-duration gamma-ray burst GRB000926. The optical transient was detected independently with the Nordic Optical Telescope and at Calar Alto 22.2 hours after the burst. At this time the magnitude of the transient was R = 19.36. The transient faded with a decay slope of about 1.7 during the first two days after which the slope increased abruptly (within a few hours) to about 2.4. The light-curve started to flatten off after about a week indicating the presence of an underlying extended object. This object was detected in a deep image obtained one month after the GRB at R=23.87+-0.15 and consists of several compact knots within about 5 arcsec. One of the knots is spatially coincident with the position of the OT and hence most likely belongs to the host galaxy. Higher resolution imaging is needed to resolve whether all the compact knots belong to the host galaxy or to several independent objects. In a separate paper we present a discussion of the optical spectrum of the OT, and its inferred redshift (Moller et al. in prep.).

astro-ph↗

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.

astro-ph↗

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.

astro-ph↗

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.

astro-ph↗