Young Brown Dwarfs in the Core of the W3 Main Star-Forming Region

We present the results of deep and high-resolution (FWHM ~ 0".35) JHK NIR observations with the Subaru telescope, to search for very low mass young stellar objects (YSOs) in the W3 Main star-forming region. The NIR survey covers an area of ~ 2.6 arcmin^2 with 10-sigma limiting magnitude exceeding 20 mag in the JHK bands. The survey is sensitive enough to provide unprecedented details in W3 IRS 5 region and reveals a census of the stellar population down to objects below the hydrogen-burning limit. We construct JHK color-color (CC) and J-H/J and H-K/K color-magnitude (CM) diagrams to identify very low luminosity YSOs and to estimate their masses. Based on these CC and CM diagrams, we identified a rich population of embedded YSO candidates with infrared excesses (Class I and Class II), associated with the W3 Main region. A large number of red sources (H-K > 2) have also been detected around W3 Main. We argue that these red stars are most probably pre-main-sequence (PMS) stars with intrinsic color excesses. Based on the comparison between theoretical evolutionary models of very low-mass PMS objects with the observed CM diagram, we find there exists a substantial substellar population in the observed region. The mass function (MF) does not show the presence of cutoff and sharp turnover around the substellar limit, at least at the hydrogen-burning limit. Furthermore, the MF slope indicates that the number ratio of young brown dwarfs and hydrogen-burning stars in the W3 Main is probably higher than those in Trapezium and IC 348. The presence of mass segregation, in the sense that relatively massive YSOs lie near the cluster center, is seen. The estimated dynamical evolution time indicates that the observed mass segregation in the W3 Main may be the imprint of the star formation process.Comment: 39 pages, 15 figures. Accepted for publication in the Astrophysical Journa

Infrared study of the eta Chamaeleontis cluster and the longevity of circumstellar discs

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 MNRA

Mid-Infrared Observations of Class I/Flat-Spectrum Systems in Six Nearby Molecular Clouds

We have obtained new mid-infrared observations of 65 Class I/Flat-Spectrum (F.S.) objects in the Perseus, Taurus, Chamaeleon I/II, Rho Ophiuchi, and Serpens dark clouds. We detected 45/48 (94%) of the single sources, 16/16 (100%) of the primary components, and 12/16 (75%) of the secondary/triple components of the binary/multiple objects surveyed. The composite spectral energy distributions (SEDs) for all of our sample sources are either Class I or F.S., and, in 15/16 multiple systems, at least one of the individual components displays a Class I or F.S. spectral index. However, the occurrence of mixed pairings, such as F.S. with Class I, F.S. with Class II, and, in one case, F.S. with Class III, is surprisingly frequent. Such behaviour is not consistent with that of multiple systems among T Tauri stars (TTS), where the companion of a classical TTS also tends to be a classical TTS, although other mixed pairings have been previously observed among Class II objects. Based on an analysis of the spectral indices of the individual binary components, there appears to be a higher proportion of mixed Class I/F.S. systems (65-80%) than that of mixed Classical/Weak-Lined TTS (25-40%), demonstrating that the envelopes of Class I/ F.S. systems are rapidly evolving during this evolutionary phase. We report the discovery of a steep spectral index secondary companion to ISO-ChaI 97, detected for the first time via our mid-infrared observations. In our previous near- infrared imaging survey of binary/multiple Class I/F.S. sources, ISO-ChaI 97 appeared to be single. With a spectral index of Alpha >= 3.9, the secondary component of this system is a member of a rare class of very steep spectral index objects, those with Alpha > 3. Only three such objects have previously been reported, all of which are either Class 0 or Class I.Comment: 31 pages, 4 figures, 6 table

The Active Corona of HD 35850 (F8 V)

We present Extreme Ultraviolet Explorer spectroscopy and photometry of the nearby F8 V star HD 35850 (HR 1817). The EUVE spectra reveal 28 emission lines from Fe IX and Fe XV to Fe XXIV. The Fe XXI 102, 129 A ratio yields an upper limit for the coronal electron density, log n < 11.6 per cc. The EUVE SW spectrum shows a small but clearly detectable continuum. The line-to-continuum ratio indicates approximately solar Fe abundances, 0.8 < Z < 1.6. The resulting emission-measure distribution is characterized by two temperature components at log T of 6.8 and 7.4. The EUVE spectra have been compared with non-simultaneous ASCA SIS spectra of HD 35850. The SIS spectrum shows the same temperature distribution as the EUVE DEM analysis. However, the SIS spectral firs suggest sub-solar abundances, 0.34 < Z < 0.81. Although some of the discrepancy may be the result of incomplete X-ray line lists, we cannot explain the disagreement between the EUVE line-to-continuum ratio and the ASCA-derived Fe abundance. Given its youth (t ~ 100 Myr), its rapid rotation (v sin i ~ 50 km/s), and its high X-ray activity (Lx ~ 1.5E+30 ergs/s), HD 35850 may represent an activity extremum for single, main-sequence F-type stars. The variability and EM distribution can be reconstructed using the continuous flaring model of Guedel provided that the flare distribution has a power-law index of 1.8. Similar results obtained for other young solar analogs suggest that continuous flaring is a viable coronal heating mechanism on rapidly rotating, late-type, main-sequence stars.Comment: 32 pages incl. 14 figures and 3 tables. To appear in the 1999 April 10 issue of The Astrophysical Journa

A Mid-Infrared Study of the Young Stellar Population in the NGC 2024 Cluster

We present the results of the first broadband 10.8 um survey of the NGC 2024 cluster. The mid-infrared data were combined with our previously published JHKL photometry in order to construct spectral energy distributions for all detected sources. The main scientific goals were to investigate the nature of the young stellar objects (YSOs) in the cluster, and to examine the efficiency of detecting circumstellar disk sources from near-infrared JHKL color-color diagrams. Out of 59 sources surveyed having K-band magnitudes K < 10.5, we detected 35 (~59%) at 10 um. Combining these detections, and upper limits for the non-detections, with existing JHKL data, we identify 1 Class I, 6 flat spectrum, 28 Class II and 5 Class III sources. We find a circumstellar disk fraction for NGC 2024 of ~85% +/- 15%, which confirms earlier published suggestions that the majority, if not all, of the stars in NGC 2024 formed with disks, and these disks still exist at the present time. In addition, all but one of the disk sources identified in our survey lie in the infrared excess region of the JHKL color-color diagram for the cluster. This demonstrates that JHKL color-color diagrams are extremely efficient in identifying YSOs with disks. Of the 14 sources with K - L colors suggestive of protostellar objects, \~29% are protostellar in nature, while ~7% are true Class I YSOs. This may be due to extinction producing very red K - L colors in Class II YSOs, thus making them appear similar in color to protostars. This suggests caution must be applied when estimating the sizes and lifetimes of protostellar populations within star forming regions based on K - L colors alone. Finally, we calculate the luminosities of the Class II YSOs in NGC 2024, rho Oph and Taurus and discuss the results.Comment: 30 pages, 8 figures, paper to appear in March A

No disks around low-mass stars and brown dwarfs in the young sigma Orionis cluster?

We report on the analysis of 2MASS near-infrared data of a sample of low-mass stars and brown dwarfs in the sigma Orionis cluster. Youth and cluster membership have been spectroscopically confirmed using the Li I spectral line. We find little evidence in the JHKs colour-colour diagram for near-infrared excess emission for these cluster members. By comparison with model expectations, at most 2 out of 34 stars show (H-K) colour consistent with a near-infrared excess. This scarcity of near-infrared signatures of circumstellar disks in the lower-mass and substellar regimes of this cluster contrasts with findings in younger clusters, hinting at an age dependence of the disk frequency. Taking into account the apparent cluster age, our result supports the idea of a relatively fast (few Myr) disk dissipation and extends this conclusion to the substellar regime. We also find some evidence that, in this cluster, the disk frequency as measured by the Ks-band excess may be mass dependent.Comment: 4 pages, 2 eps figures, accepted by Astronomy and Astrophysics Letter

The Coronae of AR Lac

We observed the coronally active eclipsing binary, AR Lac, with the High Energy Transmission Grating on Chandra for a total of 97 ks, spaced over five orbits, at quadratures and conjunctions. Contemporaneous and simultaneous EUV spectra and photometry were also obtained with the Extreme Ultraviolet Explorer. Significant variability in both X-ray and EUV fluxes were observed, dominated by at least one X-ray flare and one EUV flare. We saw no evidence of primary or secondary eclipses. X-ray flux modulation was largest at high temperature, indicative of flare heating of coronal plasma. Line widths interpreted in terms of Doppler broadening suggest that both binary stellar components are active. From line fluxes obtained from total integrated spectra, we have modeled the emission measure and abundance distributions. A strong maximum was found in the differential emission measure, characterized by peaks at log T = 6.9 and 7.4, together with a weak but significant cooler maximum near log T=6.2, and a moderately strong hot tail from log T= 7.6-8.2. Coronal abundances have a broad distribution and show no simple correlation with first ionization potential. While the resulting model spectrum generally agrees very well with the observed spectrum, there are some significant discrepancies, especially among the many Fe L-lines. Both the emission measure and abundance distributions are qualitatively similar to prior determinations from other X-ray and ultraviolet spectra, indicating some long-term stability in the overall coronal structure.Comment: 31 pages, 8 figures, 3 tables; Accepted for publication in the Astrophysical Journal (tentatively October 1, 2003

Dynamics of pebbles in the vicinity of a growing planetary embryo: hydro-dynamical simulations

Understanding the growth of the cores of giant planets is a difficult problem. Recently, Lambrechts and Johansen (2012; LJ12) proposed a new model in which the cores grow by the accretion of pebble-size objects, as the latter drift towards the star due to gas drag. Here, we investigate the dynamics of pebble-size objects in the vicinity of planetary embryos of 1 and 5 Earth masses and the resulting accretion rates. We use hydrodynamical simulations, in which the embryo influences the dynamics of the gas and the pebbles suffer gas drag according to the local gas density and velocities. The pebble dynamics in the vicinity of the planetary embryo is non-trivial, and it changes significantly with the pebble size. Nevertheless, the accretion rate of the embryo that we measure is within an order of magnitude of the rate estimated in LJ12 and tends to their value with increasing pebble-size. We conclude that the model by LJ12 has the potential to explain the rapid growth of giant planet cores. The actual accretion rates however, depend on the surface density of pebble size objects in the disk, which is unknown to date.Comment: In press in Astronomy and Astrophysic

The Rotating Quantum Vacuum

We derive conditions for rotating particle detectors to respond in a variety of bounded spacetimes and compare the results with the folklore that particle detectors do not respond in the vacuum state appropriate to their motion. Applications involving possible violations of the second law of thermodynamics are briefly addressed.Comment: Plain TeX, 10 pages (to appear in PRD

Systematic detection of magnetic fields in massive, late-type supergiants

We report the systematic detection of magnetic fields in massive (M > 5 M$_\odot$) late-type supergiants, using spectropolarimetric observations obtained with ESPaDOnS at the Canada-France-Hawaii Telescope. Our observations reveal detectable Stokes V Zeeman signatures in Least-Squares Deconvolved mean line profiles in one-third of the observed sample of more than 30 stars. The signatures are sometimes complex, revealing multiple reversals across the line. The corresponding longitudinal magnetic field is seldom detected, although our longitudinal field error bars are typically 0.3 G ($1\sigma$). These characteristics suggest topologically complex magnetic fields, presumably generated by dynamo action. The Stokes V signatures of some targets show clear time variability, indicating either rotational modulation or intrinsic evolution of the magnetic field. We also observe a weak correlation between the unsigned longitudinal magnetic field and the CaII K core emission equivalent width of the active G2Iab supergiant $\beta$~Dra and the G8Ib supergiant $\epsilon$~Gem.Comment: 8 pages, 1 table, 6 figures, accepted for publication in MNRA