* Astronomy

Members Login
Post Info
TOPIC: Brown Dwarfs


L

Posts: 131433
Date:
2MASS J05352184-0546085
Permalink  
 


Title: Discovery of X-ray emission from the eclipsing brown-dwarf binary 2MASS J05352184-0546085
Authors: S. Czesla, P.C. Schneider, J.H.M.M. Schmitt

The eclipsing brown-dwarf binary system 2MASS J05352184-0546085 is a case sui generis. For the first time, it allows a detailed analysis of the individual properties of young brown dwarfs, in particular, masses, and radii, and the temperature ratio of the system components can be determined accurately. The system shows a "temperature reversal" with the more massive component being the cooler one, and both components are found to be active. We analyse X-ray images obtained by Chandra and XMM-Newton containing 2MASS J05352184-0546085 in their respective field of view. The Chandra observatory data show a clear X-ray source at the position of 2MASS J05352184-0546085, whereas the XMM-Newton data suffer from contamination from other nearby sources, but are consistent with the Chandra detection. No indications of flaring activity are found in either of the observations (together about 70 ks), and we thus attribute the observed flux to quiescent emission. With an X-ray luminosity of 3*10^{28} erg/s we find an L_X/L_{bol}-ratio close to the saturation limit of 10^{-3} and an L_{X}/L_{H\alpha}-ratio consistent with values obtained from low-mass stars. The X-ray detection of 2MASS J05352184-0546085 reported here provides additional support for the interpretation of the temperature reversal in terms of magnetically suppressed convection, and suggests that the activity phenomena of young brown dwarfs resemble those of their more massive counterparts.

Read more (304kb, PDF)

__________________


L

Posts: 131433
Date:
RE: Brown Dwarfs
Permalink  
 


The systematics of celestial bodies apparently needs to be revised. Researchers at the Argelander Institute of Astronomy of the University of Bonn have discovered that brown dwarfs need to be treated as a separate class in addition to stars and planets.
To date they had been merely regarded as stars which were below normal size. However, they may well be stellar miscarriages. The astronomers are publishing their results in the journal Monthly Notices of the Royal Astronomical Society.
Brown dwarfs (or BDs) are what scientists call objects which populate the galaxies apart from the stars. Unlike the latter, they cannot develop high-yield hydrogen fusion as in the interior of our sun due to their low mass (less than about 8% of the suns mass). But in addition to this brown dwarfs and stars also seem to be different in their mating behaviour.
Stars often occur in pairs, which dance around each other. The intimacy which this dance involves, however, varies a great deal: sometimes the gap is smaller than one radius of the Earths orbit (also known as Astronomical Unit or AU). However, the two partners can also keep apart by as much as many thousands of AUs.

"Things are different with brown dwarfs. The orbital radiuses of BD pairs are cut off above about 15 AUs; BD pairs with greater distances are the exception" - astrophysicist Ingo Thies of the Bonn Argelander Institute of Astronomy.

What is more, there are hardly any mixed pairs consisting of suns and brown dwarfs far fewer than expected. This phenomenon is also known as brown dwarf desert.

"According to the classical model there ought not to be these differences. According to this both brown dwarfs and stars ought to emerge from interstellar clouds of gas which become concentrated because of the attraction of their mass. But if this was the case, these celestial bodies should behave in similar ways" - Professor Pavel Kroupa of the Argelander Institute.

Despite this contradiction the astronomic community has previously stuck to the theory of a joint origin. However, Ingo Thies and Pavel Kroupa have now shown empirically for the first time that brown dwarfs must be seen as a class of objects which is separate from the stars.

"For this we analysed the masses of newly born stars. This revealed a jump in the distribution of mass which makes the division in the stellar population apparent" - Ingo Thies.

But how are brown dwarfs born? As long ago as 2001 the Danish researcher Bo Reipurth, Britains Cathie Clarke and the Spanish astronomer Eduardo Delgado-Donate had the idea that brown dwarfs could be interpreted as stellar miscarriages: a system consisting of three embryonic stars disintegrates due to the mutual attraction of masses, and the lightest object is catapulted out of the system. The physical mechanism itself has long been known: even the US light space probes Pioneer and Voyager were hurled off onto their voyage of no return by the planets gravity.
Another possibility would be that brown dwarfs form in the outermost regions of emergent stars and become separated from them. This can, for example, occur as the result of a close encounter with a third star. Since almost all stars are born in star clusters, such encounters are not unusual. It is also possible that both scenarios of cosmic miscarriages take place.
Both theories predict that brown dwarfs can only emerge at the birth of stars similar to the situation with planets, incidentally. Thus there are presumably three quite different celestial bodies: planets, brown dwarfs and stars.

Source University of Bonn

Title: A discontinuity in the low-mass IMF - the case of high multiplicity
Authors: I. Thies, P. Kroupa

The empirical binary properties of brown dwarfs (BDs) differ from those of normal stars suggesting BDs form a separate population. Recent work by Thies and Kroupa revealed a discontinuity of the initial mass function (IMF) in the very-low-mass star regime under the assumption of a low multiplicity of BDs of about 15 per cent. However, previous observations had suggested that the multiplicity of BDs may be significantly higher, up to 45 per cent. This contribution investigates the implication of a high BD multiplicity on the appearance of the IMF for the Orion Nebula Cluster, Taurus-Auriga, IC 348 and the Pleiades. We show that the discontinuity remains pronounced even if the observed MF appears to be continuous, even for a BD binary fraction as high as 60%. We find no evidence for a variation of the BD IMF with star-forming conditions. The BD IMF has a power-law index alpha = +0.3 and about 2 BDs form per 10 low-mass stars assuming equal-mass pairing of BDs.

Read more (219kb, PDF)

__________________


L

Posts: 131433
Date:
Permalink  
 

Title: The Evolution of L and T Dwarfs in Colour-Magnitude Diagrams
Authors: D. Saumon (1), M. S. Marley (2) ((1) Lanl, (2) NASA Ames)

We present new evolution sequences for very low mass stars, brown dwarfs and giant planets and use them to explore a variety of influences on the evolution of these objects. We compare our results with previous work and discuss the causes of the differences and argue for the importance of the surface boundary condition provided by atmosphere models including clouds.
The L- to T-type ultracool dwarf transition can be accommodated within the Ackerman & Marley (2001) cloud model by varying the cloud sedimentation parameter. We develop a simple model for the evolution across the L/T transition. By combining the evolution calculation and our atmosphere models, we generate colours and magnitudes of synthetic populations of ultracool dwarfs in the field and in galactic clusters. We focus on near infrared colour- magnitude diagrams (CMDs) and on the nature of the "second parameter" that is responsible for the scatter of colours along the Teff sequence. Variations in metallicity and cloud parameters, unresolved binaries and possibly a relatively young population all play a role in defining the spread of brown dwarfs along the cooling sequence. We find that the transition from cloudy L dwarfs to cloudless T dwarfs slows down the evolution and causes a pile up of substellar objects in the transition region, in contradiction with previous studies. We apply the same model to the Pleiades brown dwarf sequence. Taken at face value, the Pleiades data suggest that the L/T transition occurs at lower Teff for lower gravity objects. The simulated populations of brown dwarfs also reveal that the phase of deuterium burning produces a distinctive feature in CMDs that should be detectable in ~50-100 Myr old clusters.

Read more (769kb, PDF)

__________________


L

Posts: 131433
Date:
Permalink  
 

Title: Proper motions of field L and T dwarfs -II
Authors: S.L. Casewell, R.F. Jameson, M.R. Burleigh

By using images taken with WFCAM on UKIRT and SofI on the NTT and combining them with 2MASS we have measured proper motions for 126 L and T dwarfs in the dwarf archive. Two of these L dwarfs appear to have M dwarf common proper motion companions, and 2 also appear to be high velocity dwarfs, indicating possible membership of the thick disc. We have also compared the motion of these 126 objects to that of numerous moving groups, and have identified new members of the Hyades, Ursa Major and Pleiades moving groups. These new objects, as well as those identified in Jameson et al. (2008) have allowed us to refine the L dwarf sequence for Ursa Major that was defined by Jameson et al. (2008).

Read more (30kb. PDF)

__________________


L

Posts: 131433
Date:
HN Peg B
Permalink  
 


Title: HN Peg B: A Test of Models of the L to T Dwarf Transition
Authors: S. K. Leggett, D. Saumon, Loic Albert, Michael. C. Cushing, Michael C. Liu, K. L. Luhman, M. S. Marley, J. Davy Kirkpatrick, Thomas L. Roellig, K. N. Allers

Luhman and collaborators recently discovered an early-T dwarf companion to the G0 dwarf star HN Peg, using Spitzer Infrared Array Camera (IRAC) images. Companionship was established on the basis of the common proper motion inferred from 1998 Two Micron All Sky Survey images and the 2004 IRAC images. In this paper we present new near-infrared imaging data which confirms the common proper motion of the system. We also present new 3 - 4 um spectroscopy of HN Peg B, which provides tighter constraints on both the bolometric luminosity determination and the comparison to synthetic spectra. New adaptive optics imaging data are also presented, which shows the T dwarf to be unresolved, providing limits on the multiplicity of the object. We use the age, distance and luminosity of the solar-metallicity T dwarf to determine its effective temperature and gravity, and compare synthetic spectra with these values, and a range of grain properties and vertical mixing, to the observed 0.8 - 4.0 um spectra and mid-infrared photometry. We find that models with temperature and gravity appropriate for the older end of the age range of the system (0.5 Gyr) can do a reasonable job of fitting the data, but only if the photospheric condensate cloud deck is thin, and if there is significant vertical mixing in the atmosphere. Dwarfs such as HN Peg B, with well-determined metallicity, radius, gravity and temperature will allow development of dynamical atmosphere models, leading to the solution of the puzzle of the L to T dwarf transition.

Read more (74kb, PDF)

__________________


L

Posts: 131433
Date:
RE: Brown Dwarfs
Permalink  
 


Title: Mapping the Shores of the Brown Dwarf Desert I.: Upper Scorpius
Authors: Adam L. Kraus, Michael J. Ireland (Caltech), Frantz Martinache, James P. Lloyd (Cornell)

We present the results of a survey for stellar and substellar companions to 82 young stars in the nearby OB association Upper Scorpius. This survey used nonredundant aperture-mask interferometry to achieve typical contrast limits of DeltaK~5-6 at the diffraction limit, revealing 12 new binary companions that lay below the detection limits of traditional high-resolution imaging; we also summarize a complementary snapshot imaging survey that discovered 7 directly resolved companions. The overall frequency of binary companions (~35+5/-4% at separations of 6-435 AU) appears to be equivalent to field stars of similar mass, but companions could be more common among lower-mass stars than for the field. The companion mass function has statistically significant differences compared to several suggested mass functions for the field, and we suggest an alternate log-normal parameterisation of the mass-function. Our survey limits encompass the entire brown dwarf mass range, but we only detected a single companion that might be a brown dwarf; this deficit resembles the so-called "brown dwarf desert" that has been observed by radial-velocity planet searches. Finally, our survey's deep detection limits extend into the top of the planetary mass function, reaching 8-12 MJup for half of our sample. We have not identified any planetary companions at high confidence (>99.5%), but we have identified four candidate companions at lower confidence (>97.5%) that merit additional followup to confirm or disprove their existence.

Read more (209kb, PDF)

__________________


L

Posts: 131433
Date:
Permalink  
 

Title: Brown dwarfs and very low mass stars in the Hyades cluster : a dynamically evolved mass function
Authors: J. Bouvier, T. T. Kendall, G. Meeus, L. Testi, E. Moraux, J.R. Stauffer, D. James, J.-C. Cuillandre, J. Irwin, M.J. McCaughrean, I. Baraffe, E. Bertin

We conducted a search for brown dwarfs (BDs) and very low mass (VLM) stars in the 625 Myr-old Hyades cluster in order to derive the cluster's mass function across the stellar-substellar boundary. We performed a deep (I=23, z=22.5) photometric survey over 16 sq.deg. around the cluster centre, followed up with K-band photometry to measure the proper motion of candidate members, and optical and near-IR spectroscopy of probable BD and VLM members. We report the discovery of the first 2 brown dwarfs in the Hyades cluster. The 2 objects have a spectral type early-T and their optical and near-IR photometry as well as their proper motion are consistent with them being cluster members. According to models, their mass is 50 Jupiter masses at an age of 625 Myr. We also report the discovery of 3 new very low mass stellar members of the cluster, and confirm the membership of 16 others. We combine these results with a list of previously known cluster members to build the present-day mass function (PDMF) of the Hyades cluster from 50 Jupiter masses to 3Mo. We find the Hyades PDMF to be strongly deficient in very low mass objects and brown dwarfs compared to the IMF of younger open clusters such as the Pleiades. We interpret this deficiency as the result of dynamical evolution over the past few 100 Myr, i.e., the preferential evaporation of low mass cluster members due to weak gravitational encounters. We thus estimate that the Hyades cluster currently hosts about 10-15 brown dwarfs, while its initial substellar population may have amounted up to 150-200 members.

Read more (474kb, PDF)

__________________


L

Posts: 131433
Date:
LSR0602+3910
Permalink  
 


Title: LSR0602+3910 - DISCOVERY OF A BRIGHT NEARBY L-TYPE BROWN DWARF
Authors: SAMIR SALIM, SEBASTIEN LEPINE, R. MICHAEL RICH AND MICHAEL M. SHARA

We report the discovery of LSR0602+3910, an L dwarf of class L1. The object was initially identified by Lépine et al. (2002) as a new high proper motion star lying close to the Galactic plane. Its 2MASS J -Ks = 1.43 is consistent with an L dwarf, which we now confirm spectroscopically. In addition, we see a signature of Li I absorption, making LSR0602+3910 a brown dwarf, one of the brightest known (Ks = 10.86). Among L dwarfs it is second in brightness to the combined light of 2MASS 0746+20, a close binary system.
We see no indication that LSR0602+3910 is a binary, although high-resolution imaging will be required to confirm this. Spectroscopic and photometric distance estimates agree very well, placing LSR0602+3910 at d = 10.6±0.8 pc. LSR0602+3910 was most likely missed in previous searches because of its proximity to the plane, the region that most searches avoided. We estimate that some 40% of bright L dwarfs are missed because of this selection effect.

Read more (PDF)


LSR0602+3910
Expand (29kb, 800 x 510)

Position (2000): RA 06h 02m 30.46s Dec +39° 10' 59.2"

Brown Dwarf LSR0602+3910 in Auriga
Distance:10.6 + 0.8 pc
Magnitude (V): 20.88

LSR0602+3910.kmz
Google Sky file (1kb, kmz)

-- Edited by Blobrana at 15:35, 2008-01-12

__________________


L

Posts: 131433
Date:
RE: Brown Dwarfs
Permalink  
 


Title: A high resolution spectral atlas of brown dwarfs
Authors: A. Reiners, D. Homeier, P. H. Hauschildt, F. Allard

We present a UVES/VLT high resolution atlas of three L dwarfs and one T dwarf system, spectral classes at which most of the objects are brown dwarfs. Our atlas covers the optical region from H \alpha up to the near infrared at 1 \mu m. We present spectral details of ultra-cool atmospheres at very high resolution (R ~ 33 000) and compare the spectra to model calculations. Our comparison shows that molecular features from VO and CaH, and atomic features from Cs and Rb are reasonably well fit by current models. On the other hand, features due to TiO, CrH, and water, and atomic Na and K reveal large discrepancies between model calculations and our observations.

Read more (PDF)

__________________


L

Posts: 131433
Date:
Permalink  
 

Title: A Substellar Companion to the Intermediate-Mass Giant 11 Com
Authors: Y. J. Liu, Bun'ei Sato, G. Zhao, Kunio Noguchi, H. Wang, Eiji Kambe, Hiroyasu Ando, Hideyuki Izumiura, Y. Q. Chen, Norio Okada, Eri Toyota, Masashi Omiya, Seiji Masuda, Yoichi Takeda, Daisuke Murata, Yoichi Itoh, Michitoshi Yoshida, Eiichiro Kokubo, Shigeru Ida

We report the detection of a substellar companion orbiting the intermediate-mass giant star 11 Com (G8 III). Precise Doppler measurements of the star from Xinglong station and Okayama Astrophysical Observatory (OAO) revealed Keplerian velocity variations with an orbital period of 326.03 ± 0.32 days, a semiamplitude of 302.8 ± 2.6 m/s, and an eccentricity of 0.231 ± 0.005. Adopting a stellar mass of 2.7 ± 0.3 Solar  masses, the minimum mass of the companion is 19.4 ± 1.5 Jupiter masses, well above the deuterium burning limit, and the semimajor axis is 1.29 ± 0.05 AU. This is the first result from the joint planet search program between China and Japan aiming at revealing statistics of substellar companions around intermediate-mass giants. 11 Com b emerged from 300 targets of the planet search program at OAO. The current detection rate of a brown dwarf candidate seems to be comparable to that around solar-type stars within orbital separations of ~3 AU.

Read more  (37kb, PDF)
 


11 Com (HR 4697, HD 107383, HIP 60202) is a G8 III giant star with a V magnitude V = 4.74, a colour index B V = 0.99, and a precise astrometric parallax  pi = 9.04 ± 0.86 mas (ESA 1997).
The resulting distance is 112 pc from the sun and the absolute magnitude is MV = 0.48. Effective temperature Teff = 4742 ± 100K was derived from the B V and metallicity [Fe/H] using the empirical calibration of Alonso et al. (2001), and a bolometric correction B.C. = 0.36 was derived from the calibration of Alonso et al. (1999) depending on temperature and metallicity.

__________________
«First  <  1 2 3 4 58  >  Last»  | Page of 8  sorted by
Quick Reply

Please log in to post quick replies.



Create your own FREE Forum
Report Abuse
Powered by ActiveBoard