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We present the characterization of CRTS J055255.7$-$004426 (=THOR 42), a young eclipsing binary comprising two pre-main sequence M dwarfs (combined spectral type M3.5). This nearby (103 pc), short-period (0.859 d) system was recently proposed as a member of the $\sim$24 Myr-old 32 Orionis Moving Group. Using ground- and space-based photometry in combination with medium- and high-resolution spectroscopy, we model the light and radial velocity curves to derive precise system parameters. The resulting component masses and radii are $0.497\pm0.005$ and $0.205\pm0.002$ $\rm{M}_{\odot}$, and $0.659\pm0.003$ and $0.424\pm0.002$ $\rm{R}_{\odot}$, respectively. With mass and radius uncertainties of $\sim$1 per cent and $\sim$0.5 per cent, respectively, THOR 42 is one of the most precisely characterized pre-main sequence eclipsing binaries known. Its systemic velocity, parallax, proper motion, colour-magnitude diagram placement and enlarged radii are all consistent with membership in the 32 Ori Group. The system provides a unique opportunity to test pre-main sequence evolutionary models at an age and mass range not well constrained by observation. From the radius and mass measurements we derive ages of 22-26 Myr using standard (non-magnetic) models, in excellent agreement with the age of the group. However, none of the models can simultaneously reproduce the observed mass, radius, temperature and luminosity of the coeval components. In particular, their H-R diagram ages are 2-4 times younger and we infer masses $\sim$50 per cent smaller than the dynamical values., Comment: Accepted for publication in MNRAS. 22 pages. Tables 4 and 5 are available in full as ancillary files

An absorption feature is occasionally reported around 11 microns in astronomical spectra, including those of forming stars. Candidate carriers include water ice, polycyclic aromatic hydrocarbons (PAHs), silicon carbide, crystalline silicates or even carbonates. All are known constituents of cosmic dust in one or more types of environments, though not necessarily together. In this paper we present new ground-based 8-13 micron spectra of one evolved star, several embedded young stellar objects (YSOs) and a background source lying behind a large column of the interstellar medium (ISM) toward the Galactic Centre. Our observations, obtained at a spectral resolution of approximately 100, are compared with previous lower resolution data, as well as data obtained with the Infrared Space Observatory (ISO) on these and other targets. By presenting a subset of a larger sample our aim is to establish the reality of the feature and subsequently speculate on its carrier. All evidence points toward crystalline silicate. For instance, the 11 micron band profile is well matched with the emissivity of crystalline olivine. Furthermore, the apparent association of the absorption feature with a sharp polarisation signature in the spectrum of two previously reported cases suggests a carrier with a relatively high band strength compared to amorphous silicates. If true, this would either set back the evolutionary stage in which silicates are crystallised, either to the embedded phase or even before within the ISM, or else the silicates ejected from the outflows of evolved stars retain some of their crystalline identity during their long residence in the ISM., Comment: 36 pages, 23 figures. Accepted for publication in Monthly Notices of the Royal Astronomical Society

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 MNRAS

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)

We present a comprehensive investigation of the Epsilon Chamaeleontis association (Epsilon Cha), one of several young moving groups spread across the southern sky. We re-assess the putative membership of Epsilon Cha using the best-available proper motion and spectroscopic measurements, including new ANU 2.3-m/WiFeS observations. After applying a kinematic analysis our final membership comprises 35-41 stars from B9 to mid-M spectral types, with a mean distance of 110+/-7 pc and a mean space motion of (U,V,W)=(-10.9+/-0.8,-20.4+/-1.3,-9.9+/-1.4) km/s. Theoretical evolutionary models suggest Epsilon Cha is 3-5 Myr old, distinguishing it as the youngest moving group in the solar neighbourhood. Fifteen members show 3-22 micron spectral energy distributions attributable to circumstellar discs, including 11 stars which appear to be actively accreting. Epsilon Cha's disc and accretion fractions (29+8-6 and 32+9-7 per cent, respectively) are both consistent with a typical 3-5 Myr-old population. Multi-epoch spectroscopy reveals three M-type members with broad and highly-variable H-alpha emission as well as several new spectroscopic binaries. We reject 11 stars proposed as members in the literature and suggest they may belong to the background Cha I and II clouds or other nearby young groups. Our analysis underscores the importance of a holistic and conservative approach to assigning young stars to kinematic groups, many of which have only subtly different properties and ill-defined memberships. We conclude with a brief discussion of Epsilon Cha's connection to the young open cluster Eta Cha and the Sco-Cen OB association. Contrary to earlier studies which assumed Eta and Epsilon Cha are coeval and were born in the same location, we find the groups were separated by ~30 pc when Eta Cha formed 4-8 Myr ago in the outskirts of Sco-Cen, 1-3 Myr before the majority of Epsilon Cha members., Comment: 24 pages. Accepted for publication in MNRAS (23/7/2013). Accepted version with updated text and new discussion of the Epsilon Cha disc fraction. Table B1 available on request

In the scope of the star formation process, it is unclear how the environment shapes the initial mass function (IMF). While observations of open clusters propose a universal picture for the IMF from the substellar domain up to a few solar masses, the young association eta Chamaeleontis presents an apparent lack of low mass objects (m<0.1 Msun). Another unusual feature of this cluster is the absence of wide binaries with a separation > 50 AU. We aim to test whether dynamical evolution alone can reproduce the peculiar properties of the association assuming a universal IMF. We use a pure N-body code to simulate the dynamical evolution of the cluster for 10 Myr, and compare the results with observations. A wide range of values for the initial parameters are tested in order to identify the initial state that would most likely lead to observations. In this context we also investigate the influence of the initial binary population on the dynamics and the possibility of having a discontinuous single IMF near the transition to the brown dwarf regime. We consider as an extreme case an IMF with no low mass systems (m<0.1 Msun). The initial configurations cover a wide range of initial density, from 10^2 to 10^8 stars/pc^3, in virialized, hot and cold dynamical state. We do not find any initial state that would evolve from a universal single IMF to fit the observations. Only when starting with a truncated IMF without any very low mass systems and no wide binaries, can we reproduce the cluster core properties with a success rate of 10% at best. Pure dynamical evolution alone cannot explain the observed properties of eta Cha from universal initial conditions. The lack of brown dwarfs and very low mass stars, and the peculiar binary properties (low binary fraction and lack of wide binaries), are probably the result of the star formation process in this association. (abridged), Comment: 13 pages, 8 figures, A&A accepted

We report the discovery of two young M-dwarfs, RX J0942.7-7726 (M1) and 2MASS J09424157-7727130 (M4.5), that were found only 42 arcsec apart in a survey for pre-main sequence stars surrounding the open cluster eta Chamaeleontis. Both stars have congruent proper motions and near-infrared photometry. Medium-resolution spectroscopy reveals that they are coeval (age 8-12 Myr), codistant (100-150 pc) and thus almost certainly form a true wide binary with a projected separation of 4000-6000 AU. The system appears too old and dynamically fragile to have originated in eta Cha and a traceback analysis argues for its birth in or near the Scorpius-Centaurus OB Association. RX J0942.7-7726AB joins a growing group of wide binaries kinematically linked to Sco-Cen, suggesting that such fragile systems can survive the turbulent environment of their natal molecular clouds while still being dispersed with large velocities. Conversely, the small radial velocity difference between the stars (2.7 \pm 1.0 km/s) could mean the system is unbound, a result of the coincidental ejection of two single stars with similar velocity vectors from the OB association early in its evolution., Comment: Accepted for publication in MNRAS. 11 pages, 8 figures and 2 tables

We present a deep Spitzer/IRAC survey of the OB association IC 1795 carried out to investigate the evolution of protoplanetary disks in regions of massive star formation. Combining Spitzer/IRAC data with Chandra/ACIS observations, we find 289 cluster members. An additional 340 sources with an infrared excess, but without X-ray counterpart, are classified as cluster member candidates. Both surveys are complete down to stellar masses of about 1 Msun. We present pre-main sequence isochrones computed for the first time in the Spitzer/IRAC colors. The age of the cluster, determined via the location of the Class III sources in the [3.6]-[4.5]/[3.6] color-magnitude diagram, is in the range of 3 - 5 Myr. As theoretically expected, we do not find any systematic variation in the spatial distribution of disks within 0.6 pc of either O-type star in the association. However, the disk fraction in IC 1795 does depend on the stellar mass: sources with masses >2 Msun have a disk fraction of ~20%, while lower mass objects (2-0.8 Msun) have a disk fraction of ~50%. This implies that disks around massive stars have a shorter dissipation timescale., Comment: Accepted for publication in ApJ

The prototype of accreting, pulsating white dwarfs (GW Lib) underwent a large amplitude dwarf nova outburst in 2007. We used ultraviolet data from GALEX and ground-based optical photometry and spectroscopy to follow GW Lib for three years following this outburst. Several variations are apparent during this interval. The optical shows a superhump modulation in the months following outburst while a 19 min quasi-periodic modulation lasting for several months is apparent in the year after outburst. A long timescale (about 4 hr) modulation first appears in the UV a year after outburst and increases in amplitude in the following years. This variation also appears in the optical 2 years after outburst but is not in phase with the UV. The pre-outburst pulsations are not yet visible after 3 years, likely indicating the white dwarf has not returned to its quiescent state., Comment: 32 pages, 13 figures, to be published in The Astronomical Journal

We have identified several lithium-rich low-mass (0.08

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 Letters

We have identified several lithium-rich low-mass (0.08

We present a study of flux-calibrated low-resolution optical spectroscopy of ten stars belonging to eight systems in the ~ 5 Myr-old Epsilon Chamaeleontis (Eps Cha) pre-main-sequence (PMS) star cluster. Using synthetic broadband colours, narrow-band continuum, atomic and molecular lines derived from the spectra, we compare the Eps Cha stars to a slightly older PMS cluster, the ~ 8 Myr-old Eta Cha cluster, and to main-sequence dwarfs. Using synthetic VRI colours and other indices, we find that the relationship between broadband colours and spectroscopic temperature indicators for Eps Cha cluster members is indistinguishable from that of Gyr-old dwarfs. This result is identical to that found earlier in Eta Cha. Gravity-sensitive line indices place the cluster between the giant and dwarf sequences, and there is clear evidence that Eps Cha stars have lower surface gravity than Eta Cha stars. This result is consistent with Eps Cha being the slightly younger PMS association, a few Myr younger according to the Hertzsprung-Russell (HR) diagram placement of these two clusters and comparison with PMS evolutionary grids. Late M-type Eps Cha cluster members show a B-band flux excess of ~ 0.2 mag compared to observations of standard dwarfs, which might be related to enhanced magnetic activity. A similar level of excess B-band emission appears to be a ubiquitous feature of low mass members of young stellar populations with ages less than a few hundred Myr, a very similar timescale to the PMS phase of elevated relative X-ray luminosity., Comment: 8 figures and 3 tables It accepted for publication in Monthly Notices of the Royal Astronomical Society

The mysterious high galactic latitude cometary globule CG12 has been observed with the ACIS detector on board the Chandra X-ray Observatory. We detect 128 X-ray sources; half are likely young stars formed within the globule's head. This new population of >50 T-Tauri stars and one new embedded protostar is far larger than the previously reported few intermediate-mass and two protostellar members of the cloud. Most of the newly discovered stars have masses 0.2-0.7Mo, and 9-15% have K-band excesses from inner protoplanetary disks. X-ray properties provide an independent distance estimate consistent with CG12's unusual location >200pc above the Galactic plane. The star formation efficiency in CG12 appears to be 15-35%, far above that seen in other triggered molecular globules. The median photometric age found for the T-Tauri population is ~4Myr with a large spread of <1-20Myr and ongoing star formation in the molecular cores. The stellar age and spatial distributions are inconsistent with a simple radiation driven implosion (RDI) model, and suggest either that CG12 is an atypically large shocked globule, or it has been subject to several distinct episodes of triggering and ablation. We report a previously unnoticed group of B-type stars northwest of CG12 which may be the remnants of an OB association which produced multiple supernova explosions that could have shocked and ablated the cloud over a 15-30Myr period. HD120958 (B3e), the most luminous member of the group, may be currently driving an RDI shock into the CG12 cloud., Comment: 58 pages, 12 figures, 5 tables, submitted to ApJ 05/08/07

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 MNRAS

We have conducted a photometric study of late-type members of the TW Hydrae association (TWA) and measured the rotation periods for 16 stars in 12 systems. For TWA stars listed by Webb et al. and Sterzik et al. (TWA 1-13; led by TW Hya = TWA 1) we find a median period of 4.7 d. However, for stars that we measured in the TWA 14-19 group identified by Zuckerman et al., we find a median period of only 0.7 d. The period distributions of the two groups cannot be reconciled at the 3-sigma significance level. Using photometric arguments supported by the Hipparcos distance to HD 102458 (= TWA 19A), we find that TWA 14-19 reside at an average distance of d \approx 90 pc, spatially at the near boundary of the Lower Centaurus-Crux (LCC) subgroup of the Ophiuchus-Scorpius-Centaurus OB-star association (OSCA). Proper motions for HD 102458, TWA 14, 18 and 19B link these stars to the LCC subgroup. From Hertzsprung-Russell (H-R) diagram placement, we derive an age of \approx 17 Myr for the HD 102458 system that may be the representative age for the TWA 14-19 group. Merging various lines of evidence, we conclude that these stars form a spatially- and rotationally-distinct population of older pre-main sequence (PMS) stars, rather than being an extension of the TWA beyond those stars associated with TW Hya that have an age of \sim 10 Myr and reside at d \approx 55 pc. Instead, TWA 14-19 likely represent the population of low-mass stars still physically associated with the LCC subgroup., Comment: 9 pages, 7 figures, accepted for MNRAS

Following its 2002 February eruption, V838 Mon developed a light echo that continues to expand and evolve as light from the outburst scatters off progressively more distant circumstellar and/or interstellar material. Multi-filter images of the light echo, obtained with the South African Astronomical Observatory (SAAO) 1.0-m telescope between 2002 May and 2004 December, are analysed and made available electronically. The expansion of the light echo is measured from the images and the data compared with models for scattering by a thin sheet and a thin shell of dust. From these model results we infer that the dust is likely in the form of a thin sheet distant from the star, suggesting that the material is of interstellar origin, rather than being from earlier stages in the star's evolution. Although the fit is uncertain, we derive a stellar distance of ~ 9 kpc and a star-dust distance of ~ 5 pc, in good agreement with recent results reported from other methods. We also present JHKL and Cousins UBVRI photometry obtained at the SAAO during the star's second, third and fourth observing seasons post-outburst. These data show complex infrared colour behaviour while V838 Mon is slowly brightening in the optical., Comment: 9 pages, 6 figures - accepted for publication in MNRAS

Sensitive X-ray imaging surveys provide a new and effective tool to establish the census of pre-main sequence (PMS) stars in nearby young stellar clusters. We report here a deep Chandra X-ray Observatory observation of PMS stars in the Chamaeleon I North cloud, achieving a limiting luminosity of log L_t ~ 27 erg/s (0.5-8 keV band) in a 0.8x0.8 pc region. Of the 107 X-ray sources, 37 are associated with Galactic stars of which 27 are previously recognized cloud members. These include three PMS brown dwarfs; the protostellar brown dwarf ISO 192 has a particularly high level of magnetic activity. Followup optical photometry and spectroscopy establishes that 9-10 of the Chandra sources are probably magnetically active background stars. Several previously proposed cloud members are also inferred to be interlopers due to the absence of X-ray emission at the level expected from the L_t - K correlation. No new X-ray discovered stars were confidently found despite the high sensitivity. From these findings, we argue that the sample of 27 PMS cloud members in the Chandra field is uncontaminated and complete down to K = 12 or M ~ 0.1 Mo. The initial mass function (IMF) derived from our sample is deficient in 0.1-0.3 Mo stars compared to the IMF of the rich Orion Nebula Cluster and other Galactic populations. We can not discriminate whether this is due to different star formation processes, mass segregation, or dynamical ejection of lower mass stars., Comment: 44 page manuscript with 7 figures, to appear in the Astrophysical Journal. A version with high quality figures can be found at http://www.astro.psu.edu/users/edf/ChaIN.pdf

We report new results in our continuing study of the unique compact (1 pc extent), nearby (d = 97 pc), young (t = 9 Myr) stellar cluster dominated by the B9 star eta Chamaeleontis. An optical photometric survey spanning 1.3 x 1.3 pc adds two M5-M5.5 weak-lined T Tauri (WTT) stars to the cluster inventory which is likely to be significantly complete for primaries with masses > 0.15 M_sun. The cluster now consists of 17 primaries and approximately 9 secondaries lying within 100 AU of their primaries. The apparent distribution of 9:7:1 single:binary:triple systems shows 2-4 x higher multiplicity than in the field main sequence stars, and is comparable to that seen in other pre-main sequence (PMS) populations. The initial mass function (IMF) is consistent with that of rich young clusters and field stars. By extending the cluster IMF to lower masses, we predict 10-14 additional low mass stars with 0.08 < M < 0.15 M-sun and 10-15 brown dwarfs with 0.025 < M < 0.08 M_sun remain to be discovered. The eta Cha cluster extends the established stellar density and richness relationship for young open clusters. The radial distribution of stars is consistent with an isothermal sphere, but mass segregation is present with > 50 percent of the stellar mass residing in the inner 6 arcmin (0.17 pc). Considering that the eta Cha cluster is sparse, diffuse and young, the cluster may be an ideal laboratory for distinguishing between mass segregation that is primordial in nature, or arising from dynamical interaction processes., Comment: 10 pages, 7 figures, 2 tables, accepted for MNRAS

We present the outcomes of a Chandra X-ray Observatory snapshot study of five nearby Herbig Ae/Be (HAeBe) stars which are kinematically linked with the Oph-Sco-Cen Association (OSCA). Optical photometric and spectroscopic followup was conducted for the HD 104237 field. The principal result is the discovery of a compact group of pre-main sequence (PMS) stars associated with HD 104237 and its codistant, comoving B9 neighbor epsilon Chamaeleontis AB. We name the group after the most massive member. The group has five confirmed stellar systems ranging from spectral type B9-M5, including a remarkably high degree of multiplicity for HD 104237 itself. The HD 104237 system is at least a quintet with four low mass PMS companions in nonhierarchical orbits within a projected separation of 1500 AU of the HAeBe primary. Two of the low-mass members of the group are actively accreting classical T Tauri stars. The Chandra observations also increase the census of companions for two of the other four HAeBe stars, HD 141569 and HD 150193, and identify several additional new members of the OSCA. We discuss this work in light of several theoretical issues: the origin of X-rays from HAeBe stars; the uneventful dynamical history of the high-multiplicity HD 104237 system; and the origin of the epsilon Cha group and other OSCA outlying groups in the context of turbulent giant molecular clouds. Together with the similar eta Cha cluster, we paint a portrait of sparse stellar clusters dominated by intermediate-mass stars 5-10 Myr after their formation., Comment: Accepted for publication in the Astrophysical Journal. 32 pages and 7 figures

The unusual eruptive variable discovered in Monoceros in 2002 January underwent dramatic photometric and spectroscopic changes in the months prior to its 2002 June-August conjunction with the Sun. Optical and infrared (IR) photometry obtained at the South African Astronomical Observatory (SAAO) between 2002 January and June (JD 2452280-440) is presented here in an analysis of the star's post-outburst behaviour. The light curve indicated 3 eruptions took place in 2002 January, February and March. SAAO echelle spectra obtained in the week prior to the March maximum indicated the ejection of a new shell of material. JHKL photometry obtained during 2002 April showed the development of an IR excess due to the formation of a dust shell. The shell appears to be largely responsible for the rapid fade in the optical flux during 2002 April-May (Delta V > 6 mag within 3 weeks). Blueing of the optical colours during the decline is likely due either to the revealing of an emission line region surrounding V838 Mon, or the unveiling of the progenitor or a spatially-close early-type star., Comment: 7 pages, 7 figures - accepted for MNRAS

We have analyzed JHKL observations of the stellar population of the ~9 Myr-old eta Chamaeleontis cluster. Using infrared (IR) colour-colour and colour-excess diagrams, we find the fraction of stellar systems with near-IR excess emission is 0.60 pm 0.13 (2_sigma). This results implies considerably longer disc lifetimes than found in some recent studies of other young stellar clusters. For the classical T Tauri (CTT) and weak-lined T Tauri (WTT) star population, we also find a strong correlation between the IR excess and H_alpha emission. The IR excesses of these stars indicate a wide range of star-disc activity; from a CTT star showing high levels of accretion, to CTT - WTT transition objects with evidence for some on-going accretion, and WTT stars with weak or absent IR excesses. Of the 15 known cluster members, 4 stars with IR excesses delta(K-L) > 0.4 mag are likely experiencing on-going accretion owing to strong or variable optical emission. The resulting accretion fraction (0.27 pm 0.13; 2_sigma) shows that the accretion phase, in addition to the discs themselves, can endure for at least ~10 Myr., Comment: 7 pages, 4 figures, accepted for MNRAS

A limited-area survey of the eta Chamaeleontis cluster has identified 2 new late-type members. The more significant of these is ECHA J0843.3-7905 (= IRAS F08450-7854), a slowly-rotating (P = 12 d) M2 classical T Tauri (CTT) star with a spectrum dominated by Balmer emission. At a distance of 97 pc and cluster age of approx 9 Myr, the star is a nearby rare example of an 'old' CTT star and promises to be a rewarding laboratory for the study of disk structure and evolution in pre-main sequence (PMS) stars. The other new member is the M4 weak-lined T Tauri (WTT) star ECHA J0841.5-7853, which is the lowest mass (M = 0.2 M_sun) primary known in the cluster., Comment: 5 pages, 6 figures, accepted for MNRAS. High quality copy available at http://www.ph.adfa.edu.au/w-lawson/press/CTT2001.pdf

A young, nearby compact aggregate of X-ray emitting pre-main sequence stars was recently discovered in the vicinity of eta Cha (B8V). In this paper, we further investigate this cluster: its membership, its environs and origins. ROSAT HRI X-ray data for the cluster's T Tauri stars show high levels of magnetic activity and variability. The cluster has an anomalous X-ray luminosity function compared to other young clusters, deficient in stars with low, but detectable X-ray luminosities. This suggests that many low-mass members have escaped the surveyed core region. Photographic photometry from the USNO-A2.0 catalog indicates that additional, X-ray-quiet members exist in the cluster core region. The components of the eclipsing binary RS Cha, previously modeled in the literature as post-MS with discordant ages, are shown to be consistent with being coeval pre-MS stars. We compute the Galactic motion of the cluster from Hipparcos data, and compare it to other young stars and associations in the fourth Galactic quadrant. The kinematic study shows that the eta Cha cluster, the TW Hya association, and a new group near epsilon Cha, probably originated near the giant molecular cloud complex that formed the two oldest subgroups of the Sco-Cen OB association roughly 10-15 Myr ago. Their dispersal is consistent with the velocity dispersions seen in giant molecular clouds. A large H I filament and dust lane located near eta Cha has been identified as part of a superbubble formed by Sco-Cen OB winds and supernova remnants. The passage of the superbubble may have terminated star-formation in the eta Cha cluster and dispersed its natal molecular gas., Comment: 26 pages, 9 figures, LaTex2.09, ApJ, in press, http://etacha.as.arizona.edu/~eem/etacha/MLF00/index.html

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)