The American Astronomical Society meeting in San Diego yielded results we’ll be discussing all year. One study that comes immediately to mind (with a paper scheduled for the Astronomical Journal in April) is the work of Wei-Chun Jao and the Research Consortium on Nearby Stars (RECONS) team at Georgia State University, who have measured the distance to four stars — all of them red dwarfs — within 33 light years of the Sun. All told, the team has found 26 new neighbors within 25 parsecs (82 light-years), along with the first confirmed binaries comprising a red subdwarf and a white dwarf.
Subdwarfs are highly unusual stars, with extremely low metallicity; i.e., few elements heavier than hydrogen and helium. From a press release on the star measurements:
Another indicator that both systems are old is that each travels through the Galaxy at nearly 150 km/sec (roughly 100 miles/sec). Contrary to people, older stars like the Jupiter-sized red subdwarfs generally move faster than their younger counterparts that are still hanging around their stellar nurseries, and the two discovered systems are moving about three times faster than even middle aged stars. “It’s amazing to see two senior citizens rushing through our neighborhood'”, says Jao, “and they are carrying extremely dense baggages with them.” The white dwarf companions are small objects the size of the Earth that contain roughly half the mass of our Sun, but they are unable to burn any elemental fuel in internal reactions, so they cool down as time passes. Consequently, the two galactic fossils discovered provide a new laboratory that can be used by astronomers to understand the cooling character of white dwarfs, and to probe the earliest moments of our Galaxy’s life.
What’s going on in this work is that we now have better tools to study stars with high proper motion (more than 1 arcsecond each year). These stars are all nearby, but they have been overlooked because they are small and emit little light. And while we presently know of fewer than 3000 stars within eighty light years, some astronomers estimate that there should be 10,000 within that region of space. Finding them is important as we move toward developing the Terrestrial Planet Finder mission, the NASA spacecraft that will look for Earth-like planets. In many ways, projects like RECONS are hunting for targets that may one day be visited by our robotic probes.
One star the team worked on, DENIS 1048-3956, is only 13 light-years away and thus ranks as the 28th nearest stellar system. Some data on the star (drawn from Ken Croswell’s Pinpointing a Stellar Neighbor): “The star emits only 0.00015 percent as much visible light as the Sun and, therefore, requires 1,800 years to generate the same amount of light the Sun throws off in a 24-hour day. If the star took the Sun’s place, it would look dimmer than the Full Moon.” DENIS 1048-3956 is either a red or a brown dwarf, and a known emitter of radio flares that may be a consequence of its rapid rotation.
A map of the Sun’s 25 nearest neighbors is available in Nearest 25 Star Systems at the RECONS site. Among other good things at the site is a list of the One Hundred Nearest Star Systems and links to the Nearby Stars Database, which is based on the NStars project at Northern Arizona University.
The upcoming paper is Wei Chun-Jao, Todd Henry, et al.,”The Solar Neighborhood XIII: Parallax Results from the CTIOPI 0.9-m Program — Stars with mu >= 1″/year (MOTION Sample),” which can be seen in preprint form at the ArXiv site. RECONS works with data from the 0.9m (36in) telescope at the Cerro Tololo Interamerican Observatory in Chile.
Finding Extreme Subdwarfs
Authors: J. L. Marshall (OCIW)
(Submitted on 6 Dec 2007)
Abstract: I develop a new technique to identify M-type extreme subdwarfs (esdMs) and demonstrate that it is substantially more efficient than previous methods. I begin by obtaining spectroscopy and improved photometry of a sample of 54 late-type halo candidates using the rNLTT reduced proper motion (RPM) diagram.
From spectroscopy, I find that four of these are esdMs, three of which were previously unknown. From the improved photometry, I show that all four lie in a narrow RPM corridor that contains only 4 non-esdMs. Hence, with good photometry (i.e., without spectroscopy), it appears possible to select esdM candidates with a 50% esdM yield. This is more than an order of magnitude more efficient than previous methods.
Comments: 23 pages, 10 figures. Submitted to AJ
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0712.1044v1 [astro-ph]
Submission history
From: J. L. Marshall [view email]
[v1] Thu, 6 Dec 2007 21:12:23 GMT (254kb)
http://arxiv.org/abs/0712.1044
The planet-hosting subdwarf B star V391 Pegasi is a hybrid pulsator
Authors: R. Lutz, S. Schuh, R. Silvotti, S. Bernabei, S. Dreizler, T. Stahn, S. D. Huegelmeyer
(Submitted on 28 Jan 2009)
Abstract: A noticeable fraction of subdwarf B stars shows either short-period (p-mode) or long-period (g-mode) luminosity variations, with two objects so far known to exhibit hybrid behaviour, i.e. showing both types of modes at the same time.
The pulsating subdwarf B star V391 Pegasi (or HS2201+2610), which is close to the two known hybrid pulsators in the log g – Teff plane, has recently been discovered to host a planetary companion. In order to learn more about the planetary companion and its possible influence on the evolution of its host star (subdwarf B star formation is still not well understood), an accurate characterisation of the host star is required.
As part of an ongoing effort to significantly improve the asteroseismic characterisation of the host star, we investigate the low-frequency behaviour of HS2201+2610. We obtained rapid high signal-to-noise photometric CCD (B-filter) and PMT (clear-filter) data at 2m-class telescopes and carried out a careful frequency analysis of the light curves. In addition to the previously known short-period luminosity variations in the range 342s-367s, we find a long-period variation with a period of 54min and an amplitude of 0.15 per cent.
This can most plausibly be identified with a g-mode pulsation, so that HS2201+2610 is a new addition to the short list of hybrid sdB pulsators. Along with the previously known pulsating subdwarf B stars HS0702+6043 and Balloon090100001 showing hybrid behaviour, the new hybrid HS2201+2610 is the third member of this class.
This important property of HS2201+2610 can lead to a better characterisation of this planet-hosting star, helping the characterisation of its planetary companion as well. Current pulsation models cannot yet reproduce hybrid sdBV stars particularly well and improved pulsation models for this object have to include the hybrid behaviour.
Comments: 5 pages, 3 figures, accepted for publication in ‘Astronomy & Astrophysics’
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:0901.4523v1 [astro-ph.SR]
Submission history
From: Ronny Lutz [view email]
[v1] Wed, 28 Jan 2009 19:01:20 GMT (183kb)
http://arxiv.org/abs/0901.4523