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Register a new userAsteroseismological Observations of the Central Star of the Planetary Nebula NGC 1501
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We report on a global CCD time-series photometric campaign to decode the pulsations of the nucleus of the planetary nebula NGC1501. The star is hot and hydrogen-deficient, similar to the pre-white-dwarf PG 1159 stars. NGC1501 shows pulsational brightness variations of a few percent with periods ranging from 19 to 87 minutes. The variations are very complex, suggesting a pulsation spectrum that requires a long unbroken time series to resolve. Our CCD photometry of the star covers a two-week period in 1991 November, and used a global network of observatories. We obtained nearly continuous coverage over an interval of one week in the middle of the run. We have identified 10 pulsation periods, ranging from 5235 s down to 1154 s. We find strong evidence that the modes are indeed nonradial g-modes. The ratios of the frequencies of the largest-amplitude modes agree with those expected for modes that are trapped by a density discontinuity in the outer layers. We offer a model for the pulsation spectrum that includes a common period spacing of 22.3 s and a rotation period of 1.17 days; the period spacing allows us to assign a seismological mass of 0.55+/-0.03 Msun.
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About 25% of all post-AGB stars are hydrogen-deficient, e.g. the PG1159 stars with a typical abundance pattern He:C:O = 33:50:17 (by mass). Only four of about 40 known PG1159 stars exhibit H in their spectra. The exciting star of the planetary nebula A43 is one of these so-called hybrid PG1159 stars. We present preliminary results of an on-going spectral analysis by means of NLTE model-atmosphere techniques based on UV spectra obtained with FUSE, HST/GHRS, and IUE as well as on optical observations.
We present the discovery and characterisation of the post-common-envelope central star system in the planetary nebula PN G283.7$-$05.1. Deep images taken as part of the POPIPlaN survey indicate that the nebula may possess a bipolar morphology similar to other post-common-envelope planetary nebulae. Simultaneous light and radial velocity curve modelling reveals the newly discovered binary system to comprise a highly-irradiated, M-type main-sequence star in a 5.9 hour orbit with a hot pre-white-dwarf. The nebular progenitor is found to have a particularly low mass of around 0.4 M$_odot$, making PN G283.7$-$05.1 one of only a handful of candidate planetary nebulae to be the product of a common-envelope event while still on the red giant branch. Beyond its low mass, the model temperature, surface gravity and luminosity are all found to be consistent with the observed stellar and nebular spectra through comparison with model atmospheres and photoionisation modelling. However, the high temperature (T$_mathrm{eff}sim$95kK) and high luminosity of the central star of the nebula are not consistent with post-RGB evolutionary tracks.
Abell 70 is a planetary nebula (PN) whose nucleus at optical wavelengths is a G8 star with enhanced carbon and s-process abundances--a barium star. The cool star is a binary companion of the true, hot central star. In several other members of this rare class, the cool star is known to rotate with a period of only a few days. This is attributed to a spin-up that occurred due to accretion from the wind of the central star during its post-AGB PN ejection; this also accounts for the unusual chemical composition of the companions photosphere. We obtained photometry of the central star of Abell 70 on 104 nights in 2010-11, and find a periodic 2.061-day variation, but with a variable amplitude--indicating that the variability is due to starspots on a chromospherically active star. Abell 70 thus joins other members of this class in having a rapidly rotating and chemically peculiar late-type companion of the hot central star.
We have conducted a multi-wavelength study of the planetary nebula Abell 48 and give a revised classification of its nucleus as a hydrogen-deficient star of type [WN4]. The surrounding nebula has a morphology typical of PNe and importantly, is not enriched in nitrogen, and thus not the peeled atmosphere of a massive star. Indeed, no WN4 star is known to be surrounded by such a compact nebula. The ionized mass of the nebula is also a powerful discriminant between the low-mass PN and high-mass WR ejecta interpretations. The ionized mass would be impossibly high if a distance corresponding to a Pop I star was adopted, but at a distance of 2 kpc, the mass is quite typical of moderately evolved PNe. At this distance, the ionizing star then has a luminosity of ~5000 Lsolar, again rather typical for a PN central star. We give a brief discussion of the implications of this discovery for the late-stage evolution of intermediate-mass stars.
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