New low-mass members of the Octans stellar association and an updated 30-40 Myr lithium age

The Octans association is one of several young stellar moving groups recently discovered in the Solar neighbourhood, and hence a valuable laboratory for studies of stellar, circumstellar disc and planetary evolution. However, a lack of low-mass members or any members with trigonometric parallaxes means the age, distance and space motion of the group are poorly constrained. To better determine its membership and age, we present the first spectroscopic survey for new K and M-type Octans members, resulting in the discovery of 29 UV-bright K5-M4 stars with kinematics, photometry and distances consistent with existing members. Nine new members possess strong Li I absorption, which allow us to estimate a lithium age of 30-40 Myr, similar to that of the Tucana-Horologium association and bracketed by the firm lithium depletion boundary ages of the Beta Pictoris (20 Myr) and Argus/IC 2391 (50 Myr) associations. Several stars also show hints in our medium-resolution spectra of fast rotation or spectroscopic binarity. More so than other nearby associations, Octans is much larger than its age and internal velocity dispersion imply. It may be the dispersing remnant of a sparse, extended structure which includes some younger members of the foreground Octans-Near association recently proposed by Zuckerman and collaborators.Comment: Accepted for publication in MNRAS (16 pages, 5 tables

New members of the TW Hydrae Association and two accreting M-dwarfs in Scorpius-Centaurus

We report the serendipitous discovery of several young mid-M stars found during a search for new members of the 30-40 Myr-old Octans Association. Only one of the stars may be considered a possible Octans(-Near) member. However, two stars have proper motions, kinematic distances, radial velocities, photometry and Li I 6708AA measurements consistent with membership in the 8-10 Myr-old TW Hydrae Association. Another may be an outlying member of TW Hydrae but has a velocity similar to that predicted by membership in Octans. We also identify two new lithium-rich members of the neighbouring Scorpius-Centaurus OB Association (Sco-Cen). Both exhibit large 12 and 22 micron excesses and strong, variable H-alpha emission which we attribute to accretion from circumstellar discs. Such stars are thought to be incredibly rare at the ~16 Myr median age of Sco-Cen and they join only one other confirmed M-type and three higher-mass accretors outside of Upper Scorpius. The serendipitous discovery of two accreting stars hosting large quantities of circumstellar material may be indicative of a sizeable age spread in Sco-Cen, or further evidence that disc dispersal and planet formation time-scales are longer around lower-mass stars. To aid future studies of Sco-Cen we also provide a newly-compiled catalogue of 305 early-type Hipparcos members with spectroscopic radial velocities sourced from the literature.Comment: 12 pages. Accepted for publication in MNRA

First detection of a low-mass stellar halo around the young open cluster Eta Chamaeleontis

We have identified several lithium-rich low-mass (0.08<M<0.3 Msun) stars within 5.5 deg of the young open cluster Eta Chamaeleontis, nearly four times the radius of previous search efforts. Of these stars we propose 4 new probable cluster members, and 3 possible members requiring further investigation. These findings are consistent with a dynamical origin for the current configuration of the cluster, without the need to invoke an abnormal Initial Mass Function deficient in low-mass objects. Candidates were selected on the basis of DENIS and 2MASS photometry, NOMAD astrometry and extensive follow-up spectroscopy.Comment: 5 Pages. 5 Figures and 1 Table. Accepted for publication in MNRAS Letters. Higher resolution figures available at http://www.mso.anu.edu.au/~murphysj/

The characteristics of cool hydrogen deficient carbon stars

Observations of many R Coronae Borealis (RCB) stars and Hydrogen deficient Carbon (HdC) stars have been obtained to more fully understand the characteristics of these types of objects. A long term photometric programme has been undertaken to observe most of the known RCB and HdC stars, including those in the Large Magellanic Cloud. Photometry has been obtained over durations of 700-1100 d for many of these stars, permitting a more accurate analysis of the light and colour curves of these objects than has been possible with previously published data. Properties of the low amplitude variations, observed in all these stars, are described and analysed using Fourier techniques. By combining the observed periods and estimated temperatures, the latter obtained from spectra, a period temperature relationship is produced for these stars. Most stars have a period that can be identified with a radial pulsation mode from models of hydrogen deficient carbon stars. Theoretical period decrease rates for these types of stars indicate that cooler (T eff ≈ 5000 K) stars should dominate the temperature distribution. This is not observed. The reasons for this apparent lack of cooler stars are discussed. If the evolutionary models are correct, there should be many more cool hydrogen deficient carbon stars still to be discovered. Observations obtained during declines of several RCB stars are discussed. The observations of the 1988 decline of R CrB represent the first occasion where both photometric and spectroscopic observations were obtained during the onset of the decline. For those declines for which extensive photometry is available, a major division in photometric properties is observed during the early stages of the decline. This division, where the colours of the star become either bluer, or redder, may be due to a range of dust formation angles, which alters the relative flux contributions of the photosphere and chromosphere of the star. Additional aspects of two RCB stars, NSV 6708 and RY Sgr, are described in greater detail. For NSV 6708, the low radial velocity amplitude of NSV 6708 (< 4 kms-¹) causes some difficulty for the standard decline model for these stars, which assumes that the dust formation is linked to radial pulsations. For RY Sgr, observed changes in the pulsation period in the star are interpreted with an O-C analysis to the times of maxima. Only a series of linear solutions, which have lifetimes of ~ 10 yrs, satisfy the O-C residuals to within the observational uncertainty. In addition, spectra have been obtained during the shock wave event of RY Sgr, which is seen as a splitting of the absorption lines near Vmax. Low excitation lines have a greater shock amplitude, which is consistent with their formation in the upper regions of the photosphere of the highly extended atmospheres of these stars. Finally, areas where additional progress could be made on the study of these stars are discussed

A deep photometric survey of the eta Chamaeleontis cluster down to the brown dwarf - planet boundary

We report the outcome of the deep optical/infrared photometric survey of the central region (33 X 33 arcmin or 0.9 pc^2) of the eta Chamaeleontis pre-main sequence star cluster. The completeness limits of the photometry are I = 19.1, J = 18.2 and H = 17.6; faint enough to reveal low mass members down to the brown dwarf and planet boundary of ~ 13 M_Jup. We found no such low mass members in this region. Our result combined with a previous shallower (I = 17) but larger area survey indicates that low mass objects (0.013 < M/M(solar mass) < 0.075) either were not created in the eta Cha cluster or were lost due to the early dynamical history of the cluster and ejected to outside the surveyed areas.Comment: 5 pages with 4 figures, accepted by MNRA

Episodic disk accretion in the halo of the 'old' Pre-Main Sequence cluster Eta Chamaeleontis

We present multi-epoch medium-resolution observations of two M4.5 candidate members in the halo of the ~8 Myr Eta Chamaeleontis open cluster. Over six months of observations both stars exhibited variations in their H-alpha line profiles on timescales of days to months, with at least one episode of substantial activity attributable to accretion from a circumstellar disk. We derive an accretion rate ~10^-8.7 Msun/yr for this event, with a rate of ~10^-10.6 Msun/yr in quiescence. Episodic accretion like that observed here means existing surveys of accreting Weak-lined T-Tauri Stars in young clusters are likely incomplete and that gas dissipation timescales calculated from the fraction of accreting objects are underestimates.Comment: 5 pages, 5 figures, 1 table. Accepted for publication in MNRAS Letter

Re-examining the membership and origin of the € Cha association

We present a comprehensive investigation of the Ε Chamaeleontis association (Ε Cha), one of several young moving groups spread across the southern sky. We re-assess the putative membership of Ε Cha using the best available proper motion and spectrosco

The Ever Changing Circumstellar Nebula Around UW Centauri

We present new images of the reflection nebula surrounding the R Coronae Borealis Star, UW Cen. This nebula, first detected in 1990, has changed its appearance significantly. At the estimated distance of UW Cen, this nebula is approximately 0.6 ly in radius so the nebula cannot have physically altered in only 8 years. Instead, the morphology of the nebula appears to change as different parts are illuminated by light from the central star modulated by shifting thick dust clouds near its surface. These dust clouds form and dissipate at irregular intervals causing the well-known declines in the R Coronae Borealis (RCB) stars. In this way, the central star acts like a lighthouse shining through holes in the dust clouds and lighting up different portions of the nebula. The existence of this nebula provides clues to the evolutionary history of RCB stars possibly linking them to the Planetary Nebulae and the final helium shell flash stars.Comment: To be published in ApJ Letters. 5 pages, 3 figures (2 in color