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Register a new userGaia predicted brown dwarf detection rates around FGK stars in astrometry, radial velocity, and photometric transits
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Context: The current sample of known brown dwarfs (BDs) around FGK-stars is only of the order of a hundred. The ongoing ESA mission Gaia has already collected its nominal 5 yr of mission data and might operate up to 10 yr. Aims: Using detailed simulations, we estimate the number of BDs that could be discovered by Gaia astrometry, radial velocity, and photometric transits around main sequence (V) and subgiants (IV) FGK host stars for a 5 and 10-yr mission. Methods: Using a robust $Delta chi^2$ statistic we analyse the BD companion detectability from the Besanc{c}on Galaxy population synthesis model complemented by Gaia DR2 data for the bright end, using the latest Gaia performance and scanning law, and literature-based BD-parameter distributions. Results: We report here reliable detection numbers ($Delta chi^2$>50) for a 5-yr [10-yr] mission. Astrometry alone yields 28,000-42,000 [45,000-55,000] detections out to several hundred pc [>kiloparsec], with the majority around G magnitude 14-15 [14-16] and P>200 d. Gaia radial velocity time series allow detection of 830-1100 [1500-1900] mainly massive BDs (55-80 M_J), most having P <10 d. Systems with at least 3 photometric transits (S/N>3) are expected for 720-1100 [1400-2300] BDs, averaging at 4-5 [5-6] transits per source. Overlap of astrometric and radial velocity detection yield 370-410 [870-950] candidates, transit and radial velocity 17-27 [35-56], and transit and astrometric detection 1-3 [4-6]. Conclusions: Though above numbers have +/- 50% uncertainty due to the uncertain occurrence rate and period distribution of BDs around FGK host stars, Gaia detections will number in the tens-of-thousands, enlarging the current sample by at least two orders of magnitude, allowing to investigate the BD fraction and orbital architectures as a function of host stellar parameters in greater detail than every before.
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We present new 890 $mu m$ continuum ALMA observations of 5 brown dwarfs (BDs) with infrared excess in Lupus I and III -- which, in combination with 4 BDs previously observed, allowed us to study the mm properties of the full known BD disk population of one star-forming region. Emission is detected in 5 out of the 9 BD disks. Dust disk mass, brightness profiles and characteristic sizes of the BD population are inferred from continuum flux and modeling of the observations. Only one source is marginally resolved, allowing for the determination of its disk characteristic size. We conduct a demographic comparison between the properties of disks around BDs and stars in Lupus. Due to the small sample size, we cannot confirm or disprove if the disk mass over stellar mass ratio drops for BDs, as suggested for Ophiuchus. Nevertheless, we find that all detected BD disks have an estimated dust mass between 0.2 and 3.2 $M_{bigoplus}$; these results suggest that the measured solid masses in BD disks can not explain the observed exoplanet population, analogous to earlier findings on disks around more massive stars. Combined with the low estimated accretion rates, and assuming that the mm-continuum emission is a reliable proxy for the total disk mass, we derive ratios of $dot{M}_{mathrm{acc}} / M_{mathrm{disk}}$ significantly lower than in disks around more massive stars. If confirmed with more accurate measurements of disk gas masses, this result could imply a qualitatively different relationship between disk masses and inward gas transport in BD disks.
We present the results of ALMA band 7 observations of dust and CO gas in the disks around 7 objects with spectral types ranging between M5.5 and M7.5 in Upper Scorpius OB1, and one M3 star in Ophiuchus. We detect unresolved continuum emission in all but one source, and the $^{12}$CO J=3-2 line in two sources. We constrain the dust and gas content of these systems using a grid of models calculated with the radiative transfer code MCFOST, and find disk dust masses between 0.1 and 1 M$_oplus$, suggesting that the stellar mass / disk mass correlation can be extrapolated for brown dwarfs with masses as low as 0.05 M$_odot$. The one disk in Upper Sco in which we detect CO emission, 2MASS J15555600, is also the disk with warmest inner disk as traced by its H - [4.5] photometric color. Using our radiative transfer grid, we extend the correlation between stellar luminosity and mass-averaged disk dust temperature originally derived for stellar mass objects to the brown dwarf regime to $langle T_{dust} rangle approx 22 (L_{*} /L_{odot})^{0.16} K$, applicable to spectral types of M5 and later. This is slightly shallower than the relation for earlier spectral type objects and yields warmer low-mass disks. The two prescriptions cross at 0.27 L$_odot$, corresponding to masses between 0.1 and 0.2 M$_odot$ depending on age.
In its all-sky survey, Gaia will monitor astrometrically and photometrically millions of main-sequence stars with sufficient sensitivity to brown dwarf companions within a few AUs from their host stars and to transiting brown dwarfs on very short periods, respectively. Furthermore, thousands of detected ultra-cool dwarfs in the backyard of the Sun will have direct (absolute) distance estimates from Gaia, and for these Gaia astrometry will be of sufficient precision to reveal any orbiting companions with masses as low as that of Jupiter. Gaia observations thus bear the potential for critical contributions to many important questions in brown dwarfs astrophysics (how do they form in isolation and as companions to stars? Can planets form around them? What are their fundamental parameters such as ages, masses, and radii? What is their atmospheric physics?), and their connection to stars and planets. The full legacy potential of Gaia in the realm of brown dwarf science will be realized when combined with other detection and characterization programs, both from the ground and in space.
Studying the accretion process in very low-mass objects has important implications for understanding their formation mechanism. Many nearby late-M dwarfs that have previously been identified in the field are in fact young brown dwarf members of nearby young associations. Some of them are still accreting. They are therefore excellent targets for further studies of the accretion process in the very low-mass regime at different stages. We aim to search for accreting young brown dwarf candidates in a sample of 85 nearby late-M dwarfs. Using photometric data from DENIS, 2MASS, and WISE, we constructed the spectral energy distribution of the late-M dwarfs based on BT-Settl models to detect infrared excesses. We then searched for lithium and H$alpha$ emission in candidates that exhibit infrared excesses to confirm their youth and the presence of accretion. Among the 85 late-M dwarfs, only DENIS-P J1538317$-$103850 (M5.5) shows strong infrared excesses in WISE bands. The detection of lithium absorption in the M5.5 dwarf and its Gaia trigonometric parallax indicate an age of $sim$1 Myr and a mass of 47 $M_{rm J}$. The H$alpha$ emission line in the brown dwarf shows significant variability that indicates sporadic accretion. This 1 Myr-old brown dwarf also exhibits intense accretion bursts with accretion rates of up to $10^{-7.9}$$M_{odot}$ yr$^{-1}$. Our detection of sporadic accretion in one of the youngest brown dwarfs might imply that sporadic accretion at early stages could play an important role in the formation of brown dwarfs. Very low-mass cores would not be able to accrete enough material to become stars, and thus they end up as brown dwarfs.
Debris discs are a consequence of the planet formation process and constitute the fingerprints of planetesimal systems. Their solar systems counterparts are the asteroid and Edgeworth-Kuiper belts. The aim of this paper is to provide robust numbers for the incidence of debris discs around FGK stars in the solar neighbourhood. The full sample of 177 FGK stars with d<20 pc proposed for the DUNES survey is presented. Herschel/PACS observations at 100 and 160 micron complemented with data at 70 micron, and at 250, 350 and 500 micron SPIRE photometry, were obtained. The 123 objects observed by the DUNES collaboration were presented in a previous paper. The remaining 54 stars, shared with the DEBRIS consortium and observed by them, and the combined full sample are studied in this paper. The incidence of debris discs per spectral type is analysed and put into context together with other parameters of the sample, like metallicity, rotation and activity, and age. The subsample of 105 stars with d<15 pc containing 23 F, 33 G and 49 K stars, is complete for F stars, almost complete for G stars and contains a substantial number of K stars to draw solid conclusions on objects of this spectral type. The incidence rates of debris discs per spectral type 0.26 (6 objects with excesses out of 23 F stars), 0.21 (7 out of 33 G stars) and 0.20 (10 out of 49 K stars), the fraction for all three spectral types together being 0.22 (23 out of 105 stars). Uncertainties corresponding to a 95% confidence level are given in the text for all these numbers. The medians of the upper limits of L_dust/L_* for each spectral type are 7.8E-7 (F), 1.4E-6 (G) and 2.2E-6 (K); the lowest values being around 4.0E-7. The incidence of debris discs is similar for active (young) and inactive (old) stars. The fractional luminosity tends to drop with increasing age, as expected from collisional erosion of the debris belts.
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