The SONYC survey: Towards a complete census of brown dwarfs in star forming regions

SONYC, short for "Substellar Objects in Nearby Young Clusters", is a survey program to provide a census of the substellar population in nearby star forming regions. We have conducted deep optical and near-infrared photometry in five young regions (NGC1333, rho Ophiuchi, Chamaeleon-I, Upper Sco, and Lupus-3), combined with proper motions, and followed by extensive spectroscopic campaigns with Subaru and VLT, in which we have obtained more than 700 spectra of candidate low-mass objects. We have identified and characterized more than 60 new substellar objects, among them a handful of objects with masses close to, or below the Deuterium burning limit. Through SONYC and surveys by other groups, the substellar IMF is now well characterized down to ~ 5 - 10 MJup, and we find that the ratio of the number of stars with respect to brown dwarfs lies between 2 and 6. A comprehensive survey of NGC 1333 reveals that, down to ~5MJup, free-floating objects with planetary masses are 20-50 times less numerous than stars, i.e. their total contribution to the mass budget of the clusters can be neglected.Comment: to appear in the proceedings of the conference 'Brown dwarfs come of age', May 20-24 2013, Memorie della Societa Astronomica Italian

Wavelength dependence of angular diameters of M giants: an observational perspective

We discuss the wavelength dependence of angular diameters of M giants from an observational perspective. Observers cannot directly measure an optical-depth radius for a star, despite this being a common theoretical definition. Instead, they can use an interferometer to measure the square of the fringe visibility. We present new plots of the wavelength-dependent centre-to-limb variation (CLV) of intensity of the stellar disk as well as visibility for Mira and non-Mira M giant models. We use the terms ``CLV spectra'' and ``visibility spectra'' for these plots. We discuss a model-predicted extreme limb-darkening effect (also called the narrow-bright-core effect) in very strong TiO bands which can lead to a misinterpretation of the size of a star in these bands. We find no evidence as yet that this effect occurs in real stars. Our CLV spectra can explain the similarity in visibilities of R Dor (M8IIIe) that have been observed recently despite the use of two different passbands. We compare several observations with models and find the models generally under-estimate the observed variation in visibility with wavelength. We present CLV and visibility spectra for a model that is applicable to the M supergiant alpha Ori.Comment: 16 pages with figures. Accepted by MNRA

Pulsation of M-type Mira variables with moderately different mass: search for observable mass effects

Models of M-type Miras with masses of 1 $M_\odot$ and 1.2 $M_\odot$, i.e. with envelope masses of about 0.4 $M_\odot$ and 0.6 $M_\odot$, have been constructed, and a comparison has been made of their observable properties. Geometric pulsation of continuum-forming layers is found to be little affected by the mass difference. The influence of molecular contamination of near-infrared continuum bandpasses upon interferometrically measured fit diameters ranges from undetectable to quite significant. Some pulsation cycles of the lower-mass model Mira show substantially stronger contamination than that found in any cycle of the higher-mass star. Observations which sample pulsation phase well and continuously are crucial for avoiding misinterpretations, because the assignment of absolute pulsation phases is inherently uncertain by at least 0.1 cycles, diameter changes may be strongly phase-dependent, and cycle-to-cycle variations may be substantial. In accord with expectations, we find that cycle-to-cycle variations that show up in light curves and in near-continuum diameters tend to be larger and more common in the low-mass models, leading to one possible way to discriminate mass. Two other methods, based on high-precision measurements of the pulsation amplitude and on derivation of pre-maximum effective temperatures from diameter measurements, are also discussed. High-layer features that may be strongly affected by mass are not well described by present dust-free models.Comment: Accepted for MNRAS, 8 Pages, 8 Figure

A new wide pair of cool white dwarfs in the Solar neighbourhood

We report the discovery of a wide pair (93 arcsec angular separation) of extremely cool ($T_{eff}<4000$ K) white dwarfs with a very large common proper motion ($\sim1.9$ arcsec/yr). The objects were discovered in a high proper motion survey in the poorly investigated southern sky region with $\delta<-60^{\circ}$ using SuperCOSMOS Sky Survey (SSS) data. Both objects, SSSPM J2231-7514 and SSSPM J2231-7515, show featureless optical spectra. Fits of black-body models to the spectra yield effective temperatures of 3810 K and 3600 K, respectively for the bright (V=16.60) and faint (V=16.87) component. Both degenerates are much brighter than other recent discoveries of cool white dwarfs with comparable effective temperatures and/or $B_J-R$ colours. Therefore, they should be relatively nearby objects. The comparison with other cool white dwarfs and a photometric distance determination yield distance estimates between 9 pc and 14 pc. The latter seems to be more realistic, since the good agreement of the proper motion of both components within the errors of about 8 mas/yr and the angular separation between the two stars support a distance of about 15 pc with relatively small masses of the components. With smaller distance we should be able to measure a differential proper motion due to orbital motion if the orbital plane is not strongly inclined and the present orbital velocity vector is not close to the line of sight. The space velocity based on that distance and assumptions on radial velocity makes the new pair of extremely cool white dwarfs some of the probably oldest members of the Galactic disk population, although the possibility that these objects are part of a Galactic halo dark matter component can also not yet be ruled out.Comment: 9 pages (with emulateapj5.sty), 3 figures, accepted for publication in The Astrophysical Journa

Multi-wavelength visibility measurements of the red giant R Doradus

We present visibility measurements of the nearby Mira-like star R Doradus taken over a wide range of wavelengths (650--990 nm). The observations were made using MAPPIT (Masked APerture-Plane Interference Telescope), an interferometer operating at the 3.9-m Anglo-Australian Telescope. We used a slit to mask the telescope aperture and prism to disperse the interference pattern in wavelength. We observed in R Dor strong decreases in visibility within the TiO absorption bands. The results are in general agreement with theory but differ in detail, suggesting that further work is needed to refine the theoretical models.Comment: 8 pages; SPIE Conf. 4006 "Interferometry in Optical Astronomy

Synthetic Spectra and Color-Temperature Relations of M Giants

As part of a project to model the integrated spectra and colors of elliptical galaxies through evolutionary synthesis, we have refined our synthetic spectrum calculations of M giants. After critically assessing three effective temperature scales for M giants, we adopted the relation of Dyck et al. (1996) for our models. Using empirical spectra of field M giants as a guide, we then calculated MARCS stellar atmosphere models and SSG synthetic spectra of these cool stars, adjusting the band absorption oscillator strengths of the TiO bands to better reproduce the observational data. The resulting synthetic spectra are found to be in very good agreement with the K-band spectra of stars of the appropriate spectral type taken from Kleinmann & Hall (1986) as well. Spectral types estimated from the strengths of the TiO bands and the depth of the bandhead of CO near 2.3 microns quantitatively confirm that the synthetic spectra are good representations of those of field M giants. The broad-band colors of the models match the field relations of K and early-M giants very well; for late-M giants, differences between the field-star and synthetic colors are probably caused by the omission of spectral lines of VO and water in the spectrum synthesis calculations. Here, we present four grids of K-band bolometric corrections and colors -- Johnson U-V and B-V; Cousins V-R and V-I; Johnson-Glass V-K, J-K and H-K; and CIT/CTIO V-K, J-K, H-K and CO -- for models having 3000 K < Teff < 4000 K and -0.5 < log g < 1.5. These grids, which have [Fe/H] = +0.25, 0.0, -0.5 and -1.0, extend and supplement the color-temperature relations of hotter stars presented in a companion paper (astro-ph/9911367).Comment: To appear in the March 2000 issue of the Astronomical Journal. 60 pages including 15 embedded postscript figures (one page each) and 6 embedded postscript tables (10 pages total

On the X-ray variability of magnetar 1RXS J170849.0-400910

We present a long-term X-ray flux and spectral analysis for 1RXS J170849.0-400910 using Swift/XRT spanning over 8 years from 2005-2013. We also analyze two observations from Chandra and XMM in the period from 2003-2004. In this 10-yr period, 1RXS J170849.0-400910 displayed several rotational glitches. Previous studies have claimed variations in the X-ray emission associated with some of the glitches. From our analysis we find no evidence for significant X-ray flux variations and evidence for only low-level spectral variations. We also present an updated timing solution for 1RXS J170849.0-400910, from RXTE and Swift observations, which includes a previously unreported glitch at MJD 56019. We discuss the frequency and implications of radiatively quiet glitches in magnetars.Comment: 9 pages, 2 figures, accepted for publication in Ap