HE 0230−4323 revisited: a new rapidly pulsating sdB star

HE 0230−4323 is a hot sdB star in a binary system. An earlier work demonstrated that the light curve of the system shows a strong (∼4 per cent) reflection effect and also appears to exhibit photometric variations of the type associated with the slowly pulsating class of sdB star (multiple periods in the range ∼1–2 h). In this paper, we show that HE 0230−4323 is, in fact, a rapidly pulsating sdB with at least five frequencies between 3227 and 3532 μHz (periods between 310 and 283 s). The long periods previously claimed were the result of undersampling the light curve at a time interval very close to that of the short periods. The interpretation of the very slow variation (∼0.45 d) as a reflection effect in a close binary is unaffected by these new results.Web of Scienc

EC03089−6421: A new, very rapidly pulsating sdO star

EC 03089−6421, classified sdO in the Edinburgh-Cape (EC) blue object survey, is shown to have unusually rapid pulsations with a dominant frequency near 32 mHz (amplitude ∼0.02 mag; period 31.1 s) – which appears to be strongly variable in amplitude on time-scales of hours and days – and a generally weaker frequency near 29 mHz (amplitude ∼0.004 mag; period 34.2 s), which is also variable in amplitude. This star varies at twice the frequency of any known hot subdwarf pulsator. Although the low-resolution EC spectrogram appears very similar to those of DAO stars, our analysis derives T eff = 40 200 ± 1600 K; log g = 6.25 ± 0.23 and log N(He)/N(H) = −1.63 ± 0.55; more recent spectrograms give T eff = 37 400 ± 1000 K; log g = 5.70 ± 0.13 and log N(He)/N(H) = −2.02 ± 0.17, both of which indicate that the gravity is too low for a white dwarf star, although the low temperature derived from the Balmer lines is at odds with the absence of neutral Helium and the strength of He II 4686. It is possible that EC 03089−6421 is a field analogue of the ω Cen sdO variables

Resumption of mass accretion in RS Oph

The latest outburst of the recurrent nova RS Oph occurred in 2006 February. Photometric data presented here show evidence of the resumption of optical flickering, indicating re-establishment of accretion by day 241 of the outburst. Magnitude variations of up to 0.32 mag in V band and 0.14 mag in B band on time-scales of 600–7000 s are detected. Over the two-week observational period, we also detect a 0.5 mag decline in the mean brightness, from V≈ 11.4 to 11.9, and record B≈ 12.9 mag. Limits on the mass accretion rate of [inline image] are calculated, which span the range of accretion rates modelled for direct wind accretion and Roche lobe overflow mechanisms. The current accretion rates make it difficult for thermonuclear runaway models to explain the observed recurrence interval, and this implies average accretion rates are typically higher than seen immediately post-outburst

Erratum: The helium abundance in the ejecta of U Scorpii

U Scorpii (U Sco) is a recurrent nova which has been observed in outburst on 10 occasions, most recently in 2010. We present near-infrared (near-IR) and optical spectroscopy of the 2010 outburst of U Sco. The reddening of U Sco is found to be E(B − V) = 0.14 ± 0.12, consistent with previous determinations, from simultaneous optical and near-IR observations. The spectra show the evolution of the linewidths and profiles to be consistent with previous outbursts. Velocities are found to be up to 14 000 km s−1 in broad components and up to 1800 km s−1 in narrow-line components, which become visible around day 8 due to changes in the optical depth. From the spectra we derive a helium abundance of N(He)/N(H) = 0.073 ± 0.031 from the most reliable lines available; this is lower than most other estimates and indicates that the secondary is not helium-rich, as previous studies have suggested

Sakurai's Object: characterizing the near-infrared CO ejecta between 2003 and 2007

We present observations of Sakurai's Object obtained at 1–5 μm between 2003 and 2007. By fitting a radiative transfer model to an echelle spectrum of CO fundamental absorption features around 4.7 μm, we determine the excitation conditions in the line-forming region. We find 12C/13C = 3.5+2.0−1.5, consistent with CO originating in ejecta processed by the very late thermal pulse, rather than in the pre-existing planetary nebula. We demonstrate the existence of 2.2 × 10−6≤MCO≤ 2.7 × 10−6 M⊙ of CO ejecta outside the dust, forming a high-velocity wind of 500 ± 80 km s−1. We find evidence for significant weakening of the CO band and cooling of the dust around the central star between 2003 and 2005. The gas and dust temperatures are implausibly high for stellar radiation to be the sole contributor

Follow-up observations of pulsating subdwarf B stars: Multisite campaigns on PG 1618+563B and PG 0048+091

We present follow-up observations of pulsating subdwarf B (sdB) stars as part of our efforts to resolve the pulsation spectra for use in asteroseismological analyses. This paper reports on multisite campaigns of the pulsating sdB stars PG 1618+563B and PG 0048+091. Data were obtained from observatories placed around the globe for coverage from all longitudes. For PG 1618+563B, our five-site campaign uncovered a dichotomy of pulsation states: Early during the campaign the amplitudes and phases (and perhaps frequencies) were quite variable while data obtained late in the campaign were able to fully resolve five stable pulsation frequencies. For PG 0048+091, our five-site campaign uncovered a plethora of frequencies with short pulsation lifetimes. We find them to have observed properties consistent with stochastically excited oscillations, an unexpected result for subdwarf B stars. We discuss our findings and their impact on subdwarf B asteroseismology.Comment: 50 pages including 17 figures and 10 tables. Accepted for publication in the Astrophysical Journa