The magnetic fields of hot subdwarf stars

Detection of magnetic fields has been reported in several sdO and sdB stars. Recent literature has cast doubts on the reliability of most of these detections. We revisit data previously published in the literature, and we present new observations to clarify the question of how common magnetic fields are in subdwarf stars. We consider a sample of about 40 hot subdwarf stars. About 30 of them have been observed with the FORS1 and FORS2 instruments of the ESO VLT. Here we present new FORS1 field measurements for 17 stars, 14 of which have never been observed for magnetic fields before. We also critically review the measurements already published in the literature, and in particular we try to explain why previous papers based on the same FORS1 data have reported contradictory results. All new and re-reduced measurements obtained with FORS1 are shown to be consistent with non-detection of magnetic fields. We explain previous spurious field detections from data obtained with FORS1 as due to a non-optimal method of wavelength calibration. Field detections in other surveys are found to be uncertain or doubtful, and certainly in need of confirmation. There is presently no strong evidence for the occurrence of a magnetic field in any sdB or sdO star, with typical longitudinal field uncertainties of the order of 2-400 G. It appears that globally simple fields of more than about 1 or 2 kG in strength occur in at most a few percent of hot subdwarfs, and may be completely absent at this strength. Further high-precision surveys, both with high-resolution spectropolarimeters and with instruments similar to FORS1 on large telescopes, would be very valuable

Petrology of the Cenozoic phonolites and related rocks of the Houston Creek area, Bear Lodge Mountains, Wyoming

The purpose of this study was to map and study the characteristics and origin of the Cenozoic phonolites and related rocks of the Houston Creek area, located on the southwestern flank of the Bear Lodge Mountains, 10 km northwest of Sundance, Wyoming. Mapping was conducted on a 2 1:10,000 scale over a 16 km area. Microprobe, optical, and x-ray diffraction procedures were employed to observe and describe petrographic and chemical characteristics of the rocks. The Cenozoic igneous rocks of the Houston Creek area are classified as phonolite, phonolite porphyry, trachyte porphyry, sodalite-bearing phonolite and altered felsite. These hypabyssal rock types are characterized by porphyritic volcanic textures and trachytic to pilotaxitic fabrics. They contain phenocrysts of sanidine, aegirine-augite, hauyne, sodalite, orthoclase, sphene, and melanite, with analcime, natrolite, calcite, and clays(?) occurring as major alteration products. Field relationships indicate that phonolite and trachyte porphyry were emplaced as sills within the Minnelusa Formation and phonolite porphyry was emplaced as a sill at the Deadwood-Whitewood Formation contact. Sodalite-bearing phonolite appears to have been injected as dikes along fracture zones developed in altered felsite, which comprises the intrusive core of the Bear Lodge Mountains. Major element chemistry shows that the phonolites are sodic in character and are higher in total alkalis and lower in s10 2 than trachyte porphyry. Chemical and petrographic trends indicate that these rocks resemble those of a typical alkali olivine basalt series and that the primary magma was phonolitic in composition. Granitic xenoliths, present in slightly Si02 -enriched phonolites, suggest that granitic crustal contamination may have been a major cause of igneous variation in the Houston Creek area. The magma was partly crystallized at the time of emplacement and is 0 interpreted to have had a temperature of somewhat less than 715 C. Present evidence indicates the possibility that the phonolitic magma was generated by partial melting of alkali olivine basaltic material in the lower crust. The partial melting may have been initiated by Laramide induced uplift and relief of pressure at depth, which produced an influx of alkali-rich volatiles from the underlying mantle. These volatiles served to focus beat, reduce crustal melting temperatures, and provide the magma with its alkaline character

First Attempt at Spectroscopic Detection of Gravity Modes in a Long-Period Pulsating Subdwarf B Star -- PG 1627+017

In the first spectroscopic campaign for a PG 1716 variable (or long-period pulsating subdwarf B star), we succeeded in detecting velocity variations due to g-mode pulsations at a level of 1.0-1.5 km/s.The observations were obtained during 40 nights on 2-m class telescopes in Arizona, South Africa,and Australia. The target,PG1627+017, is one of the brightest and largest amplitude stars in its class.It is also the visible component of a post-common envelope binary.Our final radial velocity data set includes 84 hours of time-series spectroscopy over a time baseline of 53 days. Our derived radial velocity amplitude spectrum, after subtracting the orbital motion, shows three potential pulsational modes 3-4 sigma above the mean noise level, at 7201.0s,7014.6s and 7037.3s.Only one of the features is statistically likely to be real,but all three are tantalizingly close to, or a one day alias of, the three strongest periodicities found in the concurrent photometric campaign. We further attempted to detect pulsational variations in the Balmer line amplitudes. The single detected periodicity of 7209 s, although weak, is consistent with theoretical expectations as a function of wavelength.Furthermore, it allows us to rule out a degree index of l= 3 or l= 5 for that mode. Given the extreme weakness of g-mode pulsations in these stars,we conclude that anything beyond simply detecting their presence will require larger telescopes,higher efficiency spectral monitoring over longer time baselines,improved longitude coverage, and increased radial velocity precision.Comment: 39 pages, 9 figures, 4 tables, ApJ accepted. See postscript for full abtrac

Hot Subdwarfs in Resolved Binaries

In the last decade or so, there have been numerous searches for hot subdwarfs in close binaries. There has been little to no attention paid to wide binaries however. The advantages of understanding these systems can be many. The stars can be assumed to be coeval, which means they have common properties. The distance and metallicity, for example, are both unknown for the subdwarf component, but may be determinable for the secondary, allowing other properties of the subdwarf to be estimated. With this in mind, we have started a search for common proper motion pairs containing a hot subdwarf component. We have uncovered several promising candidate systems, which are presented here.Comment: 6 pages, 4 figures. Proceedings of The Fourth Meeting on Hot Subdwarf Stars and Related Objects held in China, 20-24 July 2009. Accepted for publication in Astrophysics and Space Scienc

Magnetic fields in central stars of planetary nebulae?

Most of the planetary nebulae (PN) have bipolar or other non-spherically symmetric shapes. The presence of a magnetic field in the central star may be the reason for this lack of symmetry, but observational works published in the literature have so far reported contradictory results. We try to correlate the presence of a magnetic field with the departures from the spherical geometry of the envelopes of planetary nebulae. We determine the magnetic field from spectropolarimetric observations of ten central stars of planetary nebulae. The results of the analysis of the observations of four stars was previously presented and discussed in the literature, while the observations of six stars, plus additional measurements for a star previously observed, are presented here for the first time. All our determinations of magnetic field in the central planetary nebulae are consistent with null results. Our field measurements have a typical error bar of 150-300 G. Previous spurious field detections obtained with FORS were probably due to the use of different wavelength calibration solutions for frames obtained at different position angles of the retarder waveplate. Currently, there is no observational evidence for the presence of magnetic fields with a strength of the order of hundreds Gauss or higher in the central stars of planetary nebulae.Comment: 5 pages, 2 figures, accepted for publication by Astronomy and Astrophysic

Elementary Forms of the Metaphorical Life : Tropes at Work in Durkheim’s Theory of the Religious

Peer reviewedPostprin

The Clusters AgeS Experiment (CASE). I. V209 omega Cen - An Eclipsing Post-Common Envelope Binary in the Globular Cluster omega Cen

We use photometric and spectroscopic observations of the detached eclipsing binary V209 omega Cen to derive the masses, radii, and luminosities of the component stars. The system exhibits total eclipses and, based on the measured systemic velocity and the derived distance, is a member of the globular cluster omega Cen. We obtain 0.945 +/- 0.043 Msun, 0.983 +/- 0.015 Rsun and 6.68 +/- 0.88 Lsun for the cooler, but larger and more luminous primary component. The secondary component has 0.144 +/- 0.008 Msun, 0.425 +/- 0.008 Rsun and 2.26 +/- 0.28 Lsun. The effective temperatures are estimated at 9370 K for the primary and at 10866 K for the secondary. On the color-magnitude diagram of the cluster, the primary component occupies a position between the tip of the blue straggler region and the extended horizontal branch while the secondary component is located close to the red border of the area occupied by hot subdwarfs. However, its radius is too large and its effective temperature is too low for it to be an sdB star. We propose a scenario leading to the formation of a system with such unusual properties with the primary component ``re-born'' from a former white dwarf which accreted a new envelope through mass transfer from its companion. The secondary star has lost most of its envelope while starting its ascent onto the sub-giant branch. It failed to ignite helium in its core and is currently powered by a hydrogen burning shell.Comment: 24 pages, 9 figures, AJ, in pres

Hypervelocity Stars. I. The Spectroscopic Survey

We discuss our targeted search for hypervelocity stars (HVSs), stars traveling with velocities so extreme that dynamical ejection from a massive black hole is their only suggested origin. Our survey, now half complete, has successfully identified a total of four probable HVSs plus a number of other unusual objects. Here we report the most recently discovered two HVSs: SDSS J110557.45+093439.5 and possibly SDSS J113312.12+010824, traveling with Galactic rest-frame velocities at least +508+-12 and +418+-10 km/s, respectively. The other late B-type objects in our survey are consistent with a population of post main-sequence stars or blue stragglers in the Galactic halo, with mean metallicity [Fe/H]=-1.3 and velocity dispersion 108+-5 km/s. Interestingly, the velocity distribution shows a tail of objects with large positive velocities that may be a mix of low-velocity HVSs and high-velocity runaway stars. Our survey also includes a number of DA white dwarfs with unusually red colors, possibly extremely low mass objects. Two of our objects are B supergiants in the Leo A dwarf, providing the first spectroscopic evidence for star formation in this dwarf galaxy within the last ~30 Myr.Comment: 10 pages, uses emulateapj, accepted by Ap

Observing Strategies for the Detection of Jupiter Analogs

To understand the frequency, and thus the formation and evolution, of planetary systems like our own solar system, it is critical to detect Jupiter-like planets in Jupiter-like orbits. For long-term radial-velocity monitoring, it is useful to estimate the observational effort required to reliably detect such objects, particularly in light of severe competition for limited telescope time. We perform detailed simulations of observational campaigns, maximizing the realism of the sampling of a set of simulated observations. We then compute the detection limits for each campaign to quantify the effect of increasing the number of observational epochs and varying their time coverage. We show that once there is sufficient time baseline to detect a given orbital period, it becomes less effective to add further time coverage-rather, the detectability of a planet scales roughly as the square root of the number of observations, independently of the number of orbital cycles included in the data string. We also show that no noise floor is reached, with a continuing improvement in detectability at the maximum number of observations N = 500 tested here.Peer reviewe

Interaction between Cape hake spawning and the circulation in the northern Benguela upwelling ecosystem

Cape hake in Namibian waters are demersal and mesopelagic spawners, spawning peaking offshore between 100 and 400 m deep, depending on local environmental conditions. The cross-shelf circulation, low-oxygen layers and mesoscale gyres are three important environmental factors influencing hake spawning behaviour and subsequent transport of the spawning products. Normally, hake spawn offshore near the bottom at depths of 150–400 m. However, during one cruise, spawning was concentrated below several subsurface mesoscale gyres, resulting in reduced dispersion of the eggs and larvae. When the low-oxygen layer above the bottom is pronounced, hake spawning has been observed close to the top of the layer at oxygen concentrations as low as 0.2–0.3 ml l-1. The relatively small size of the eggs and their high specific gravity make them ascend quite slowly from the spawning depths, 10–40 m per day. Consequently, hake eggs spawned deeper than 200 m hatch before they reach the upper mixed layer. The newly hatched larvae are relatively undeveloped, without functional eyes or mouth, and display little swimming activity during their first hours, but laboratory observations have revealed subsequent periods of downward swimming activity. Based on current field observations, on buoyancy measurements of eggs and larvae and on observed larval behaviour, it is concluded that hake eggs and larvae are transported onshore by features of the upwelling subsurface circulation that compensate for offshore movement of surface water. This may be the basic mechanism concentrating early juvenile hake nearshore. Spawning activity near the low-oxygen layer might be a behavioural adaptation to minimize egg predation, because few other species are expected to survive such low concentrations of oxygen.Keywords: buoyancy, eggs, larvae, recruitment processes, retention, vertical distributionAfrican Journal of Marine Science 2001, 23: 317–33