Li abundance/surface activity connections in solar-type Pleiades

The relation between the lithium abundance, <i>A<sub>Li</sub></i>, and photospheric activity of solar-type Pleiads is investigated for the first time via acquisition and analysis of B and V-band data. Predictions of activity levels of target stars were made according to the <i>A<sub>Li</sub></i>/ (B-V) relation and then compared with new CCD photometric measurements. Six sources behaved according to the predictions while one star (HII 676), with low predicted activity, exhibited the largest variability of the study; another star (HII 3197), with high predicted activity, was surprisingly quiet. Two stars displayed non-periodic fadings, this being symptomatic of orbiting disk-like structures with irregular density distributions. Although the observation windows were not ideal for rotational period detection, some periodograms provided possible values; the light-curve obtained for HII 1532 is consistent with that previously recorded

Explaining UXOR variability with self-shadowed disks

In this Letter we propose a new view on UX Orionis type variability. The idea is based on the earlier proposal by various authors that UXORs are nearly-edge-on disks in which hydrodynamic fluctuations could cause clumps of dust and gas to cross the line of sight. However, because the standard disk models have a flaring geometry, it is mostly the outer regions of the disk that obscure the star. The time scales for such obscuration events would be too long to match the observed time scales of weeks to months. Recent 2-D self-consistent models of Herbig Ae/Be protoplanetary disks (Dullemond et al. 2002,2003 henceforth D02/DD03), however, have indicated that for Herbig Ae/Be star disks there exists, in addition to the usual flared disks, also a new class of disks: those that are fully self-shadowed. Only their puffed-up inner rim (at the dust evaporation radius) is directly irradiated by the star, while the disk at larger radius resides in the shadow of the rim. For these disks there exist inclinations at which the line of sight towards the star skims the upper parts of the puffed-up inner rim, while passing high over the surface of outer disk regions. Small hydrodynamic fluctuations in the puffed-up inner rim could then be held responsible for the extinction events seen in UXORs. If this idea is correct, it makes a prediction for the shape of the SEDs of these stars. It was shown by D02/DD03 that flared disks have a strong far-IR excess and can be classified as `group I' (in the classification of Meeus et al. 2001), while self-shadowed disks have a relatively weak far-IR excess and are classified as `group II'. Our model therefore predicts that UXORs belong to the `group II' sources. We show that this correlation is indeed found within a sample of 86 Herbig Ae/Be stars.Comment: Accepted for publication in ApJ Letters (a few lines added to original version to accommodate comments of referee

Blue Variable Stars from the MACHO database I: Photometry and Spectroscopy of the LMC sample

We present the photometric properties of 1279 blue variable stars within the LMC. Photometry is derived from the MACHO database. The lightcurves of the sample exhibit a variety of quasi-periodic and aperiodic outburst behavior. A characteristic feature of the photometric variation is that the objects are reddest when at maximum outburst. A subset of 102 objects were examined spectroscopically. Within this subset, 91% exhibited Balmer emission in at least one epoch, in some cases with spectacular spectral variability. The variability observed in the sample is consistent with the establishment and maintenance of the Be phenomenon.Comment: 19 pages, AJ accepte

Chiral and herringbone symmetry breaking in water-surface monolayers

We report the observation from monolayers of eicosanoic acid in the L′2 phase of three distinct out-of-plane first-order diffraction peaks, indicating molecular tilt in a nonsymmetry direction and hence the absence of mirror symmetry. At lower pressures the molecules tilt in the direction of their nearest neighbors. In this region we find a structural transition, which we tentatively identify as the rotator-herringbone transition L2d−L2h

Inner disc rearrangement revealed by dramatic brightness variations in the young star PV Cep

Young Sun-like stars at the beginning of the pre-main sequence (PMS) evolution are surrounded by accretion discs and remnant protostellar envelopes. Photometric and spectroscopic variations of these stars are driven by interactions of the star with the disc. Time scales and wavelength dependence of the variability carry information on the physical mechanisms behind these interactions. We conducted multi-epoch, multi-wavelength study of PV Cep, a strongly variable, accreting PMS star. By combining our own observations from 2004-2010 with archival and literature data, we show that PV Cep started a spectacular fading in 2005, reaching an I_C-band amplitude of 4 mag. Analysis of variation of the optical and infrared fluxes, colour indices, and emission line fluxes suggests that the photometric decline in 2005-2009 resulted from an interplay between variable accretion and circumstellar extinction: since the central luminosity of the system is dominated by accretion, a modest drop in the accretion rate could induce the drastic restructuring of the inner disc. Dust condensation in the inner disc region might have resulted in the enhancement of the circumstellar extinction.Comment: 11 pages, 4 figures, accepted for publication by MNRAS. 3 online tables adde

Relation between the luminosity of young stellar objects and their circumstellar environment

We present a new model-independent method of comparison of NIR visibility data of YSOs. The method is based on scaling the measured baseline with the YSO's distance and luminosity, which removes the dependence of visibility on these two variables. We use this method to compare all available NIR visibility data and demonstrate that it distinguishes YSOs of luminosity >1000L_sun (low-L) from YSOs of <1000L_sun (high-L). This confirms earlier suggestions, based on fits of image models to the visibility data, for the difference between the NIR sizes of these two luminosity groups. When plotted against the ``scaled'' baseline, the visibility creates the following data clusters: low-L Herbig Ae/Be stars, T Tauri stars, and high-L Herbig Be stars. The T Tau cluster is similar to the low-L Herbig Ae/Be cluster, which has ~7 times smaller ``scaled'' baselines than the high-L Herbig Be cluster. We model the shape and size of clusters with different image models and find that low-L Herbig stars are the best explained by the uniform brightness ring and the halo model, T Tauri stars with the halo model, and high-L Herbig stars with the accretion disk model. However, the plausibility of each model is not well established. Therefore, we try to build a descriptive model of the circumstellar environment consistent with various observed properties of YSOs. We argue that low-L YSOs have optically thick disks with the optically thin inner dust sublimation cavity and an optically thin dusty outflow above the inner disk regions. High-L YSOs have optically thick accretion disks with high accretion rates enabling gas to dominate the NIR emission over dust. Although observations would favor such a description of YSOs, the required dust distribution is not supported by our current understanding of dust dynamics.Comment: 20 pages, 12 figures, Accepted for publication in the Astrophysical Journa

Ev\u27ry Night I Cry Myself : To Sleep Over You

https://digitalcommons.library.umaine.edu/mmb-vp/4775/thumbnail.jp

The Enigmatic Young Object: Walker 90/V590 Monocerotis

We assess the evolutionary status of the intriguing object Walker 90/V590 Mon, which is located about 20 arcminutes northwest of the Cone Nebula near the center of the open cluster NGC 2264. This object, according to its most recent optical spectral type determination (B7), which we confirmed, is at least 3 magnitudes too faint in V for the cluster distance, but it shows the classical signs of a young pre-main sequence object, such as highly variable Halpha emission, Mg II emission, IR excess, UV continuum, and optical variability. We analyzed a collection of archival and original data on Walker 90, covering 45 years including photometry, imaging, and spectroscopic data ranging from ultraviolet to near-infrared wavelengths. According to star formation processes, it is expected that, as this object clears its primordial surroundings, it should become optically brighter, show a weakening of its IR excess and present decreasing line emissions. This behavior is supported by our observations and analysis, but timescales are expected to be longer than the one observed here. Based on photometric data secured in 2007, we find Walker 90 at its brightest recorded optical magnitude. We document an evolution in spectral type over the past five decades (from A2/A3 to currently B7 and as early as B4), along with a decrease in the near-infrared K fluxes. From near-infrared images secured in 2004, Walker 90 appears as a point source placing an upper limit of 0.1 arcsec for its diameter. We conclude that many observational features are explained if W90 is a flared disk system, surrounded by an inclined optically thick accretion disk.Comment: 13 pages, 19 figure

VLT/NACO adaptive optics imaging of the TY CrA system - A fourth stellar component candidate detected

We report the detection of a possible subsolar mass companion to the triple young system TY CrA using the NACO instrument at the VLT UT4 during its commissioning. Assuming for TY CrA a distance similar to that of the close binary system HD 176386, the photometric spectral type of this fourth stellar component candidate is consistent with an ~M4 star. We discuss the dynamical stability of this possible quadruple system as well as the possible location of dusty particles inside or outside the system.Comment: 4 pages, 2 figures postscrip

Low-resolution spectroscopy and spectral energy distributions of selected sources towards sigma Orionis

Aims: We investigated in detail nine sources in the direction of the young sigma Orionis cluster, which is considered a unique site for studying stellar and substellar formation. The nine sources were selected because of some peculiar properties, such as extremely red infrared colours or too strong Halpha emission for their blue optical colours. Methods: We took high-quality, low-resolution spectroscopy (R ~ 500) of the nine targets with ALFOSC at the Nordic Optical Telescope. We also re-analyzed [24]-band photometry from MIPS/Spitzer and compiled the best photometry available at the ViJHKs passbands and the four IRAC/Spitzer channels for constructing accurate spectral energy distributions covering from 0.55 to 24 mum. Results: The nine targets were classified into: one Herbig Ae/Be star with a scatterer edge-on disc, two G-type stars, one X-ray flaring, early-M, young star with chromospheric Halpha emission, one very low-mass, accreting, young spectroscopic binary, two young objects at the brown dwarf boundary with the characteristics of classical T Tauri stars, and two emission-line galaxies, one undergoing star formation, and another one whose spectral energy distribution is dominated by an active galactic nucleus. Besides, we discover three infrared sources associated to overdensities in a cold cloud in the cluster centre. Conclusions: Low-resolution spectroscopy and spectral energy distributions are a vital tool for measuring the physical properties and the evolution of young stars and candidates in the sigma Orionis cluster.Comment: Accepted for publication in A&