arXiv subjects

A. Magazzu

Publications and source records attributed to A. Magazzu.

25 records · Page 2Linked to original sources

arXiv1998

A brown dwarf candidate in the Praesepe Open Cluster

We present optical and infrared observations of RPr1, a faint (I = 21.01) and very red object (I - K = 4.57) discovered in a deep CCD survey, covering an area of 800 square arcmin of the Praesepe open cluster. A low resolution spectrum shows that RPr1 is a very late object, the latest object in Praesepe for which a spectrum has been taken to date. Our estimates give a mass between 0.063 and 0.084 solar masses, and indicate that RPr1 may turn out to be the first brown dwarf in this cluster.

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arXiv1997

New Brown Dwarfs in the Pleiades Cluster

We present intermediate and low resolution optical spectroscopy (650-915 nm) of seven faint, very red objects (20 > I >= 17.8, I-Z >= 0.5) discovered in a CCD-based IZ survey covering an area of 1 sq. deg in the central region of the Pleiades open cluster. The observed spectra show that these objects are very cool dwarfs, having spectral types in the range M6-M9. Five out of the seven objects can be considered as Pleiades members on the basis of their radial velocities, Halpha emission and other gravity sensitive atomic features like the NaI doublet at 818.3 and 819.5 nm. According to current evolutionary models the masses of these new objects range from roughly 80 MJup for the hottest in the sample down to 45 MJup for Roque 4, the coolest and faintest confirmed member. These observations prove that the cloud fragmentation process extends well into the brown dwarf realm, suggesting a rise in the initial mass function below the substellar limit.

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arXiv1996

Search for young low-mass stars in a ROSAT selected sample south of the Taurus-Auriga molecular clouds

We present results of intermediate resolution spectroscopy of 131 optical counterparts to 115 ROSAT All-Sky Survey X-ray sources south of the Taurus-Auriga dark cloud complex. These objects have been selected as candidate young stars from a total of 1084 ROSAT sources in a about 300 square degree area. We identify 30 objects as low-mass PMS stars on the basis of the Li 6708 doublet in their spectrum, a signature of their young age. All these stars have a spectral type later than F7 and show spectral characteristics typical of weak-line and post-T Tauri stars. The presence of young objects several parsecs away from the regions of ongoing star formation is discussed in the light of the current models of T Tauri dispersal.

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arXiv1996

Theoretical LTE & NLTE curves of growth for Li I lines in G-M dwarfs and subgiants

Detailed simultaneous radiative transfer-statistical equilibrium calculations have been performed for lithium in atmospheres of solar metallicity G-M dwarfs and subgiants. Model atmospheres of stars with $3500 \le \Tf \le 6000$ K, $3.0 \le \lgg \le 4.5$ have been considered. The behaviour of level populations, source functions and curves of growth for the lithium doublet at $λ670.8$~nm and two subordinate lines of astrophysical interest ($λ610.3$~nm and $λ812.6$~nm) are discussed. For the coolest atmospheres of our grid we find that the dependence of non-LTE equivalent widths and profiles on gravity is very weak. The computed LTE and non-LTE curves of growth are given in tabular form.

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arXiv1994

Lithium and the Nature of Brown Dwarf Candidates

This paper discusses the ability of Li to confirm the substellar nature of a brown dwarf candidate. Theoretical computations using different interior models agree that brown dwarfs with masses below $\sim$ 0.065 M$_{\odot}$ preserve a significant fraction of their initial Li content while for higher masses total Li depletion occurs in very short timescales. Refined spectral synthesis using brown dwarf model atmospheres shows that the Li I resonance line at 670.8 nm should produce a conspicuous feature at effective temperatures higher than 2000 K, suitable for spectroscopic detection in present brown dwarf candidates. We summarize the results of our search for Li in many of the best candidates known so far.

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arXiv1994

Constraints to the Masses of Brown Dwarf Candidates from the Lithium Test

We present intermediate dispersion (0.7-2.2 Å~pix$^{-1}$) optical spectroscopic observations aimed at applying the ``Lithium Test'' to a sample of ten brown dwarf candidates located in the general field, two in young open clusters, and two in close binaries. We find evidence for strong Li depletion in all of them, and thus infer lower mass limits of 0.065~M$_\odot$, depending only slightly ($\pm$0.005~M$_\odot$) on the interior models. None of the field brown dwarf candidates in our sample appears to be a very young (age $<$~10$^8$~yr) substellar object. For one of the faintest proper motion Pleiades members known (V=20.7) the Li test implies a mass greater than $\sim$0.08~M$_\odot$, and therefore it is not a brown dwarf. From our spectra we estimate spectral types for some objects and present measurements of Halpha emission strengths and radial velocities. Finally, we compare the positions in the H-R diagram of our sample of brown dwarf candidates with the theoretical region where Li is expected to be preserved (Substellar Lithium Region). We find that certain combinations of temperature calibrations and evolutionary tracks are consistent with the constraints imposed by the observed Li depletion in brown dwarf candidates, while others are not.

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arXiv1993

Pre-main sequence lithium burning. I. Weak T Tauri stars

We derive lithium abundances in 53 T Tauri stars (TTS), concentrating on weak-line TTS. Our study gives the following results: 1)At luminosities $\ge 0.9 L\sun$ the Li abundances are remarkably uniform, with a mean value, log~N(Li)=3.1, equal to the "cosmic" lithium abundance. 2)Significant Li depletion appears below 0.5~L$\sun$ in the mass range 0.9-0.2 M$\sun$ and increases towards lower luminosities. At the lower mass end (0.4-0.2 M$\sun$), the observed luminosity of the Li burning turning point is about a factor 4 higher than predicted by the models. At masses 1.2-1.0 M$\sun$ the observations imply less PMS Li burning than theoretically expected. Moreover we show that: 1)Low Li abundances appear only among stars with low $v\sin{i}$. Fast rotators with masses around 0.8~M$\sun$ do not show evidence for strong Li depletion towards lower luminosities as slow rotators do. 2)In a sample restricted to only K5-K7 stars we find that the angular momentum spread before Li burning begins is larger than a factor 10. Lithium depletion associated to angular momentum loss during PMS evolution is not required to explain the observed abundances. The efficiency of PMS Li burning in the mass range 0.9-0.7~M$\sun$ is reduced in the presence of rapid rotation.

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