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N. Phan-Bao

Publications and source records attributed to N. Phan-Bao.

30 records · Page 2Linked to original sources
arXiv2007

Discovery of new nearby L and late-M dwarfs at low Galactic latitude from the DENIS database

We report new nearby L and late-M dwarfs (d_phot <= 30 pc) discovered in our search for nearby ultracool dwarfs (I-J >= 3.0, later than M8.0) at low Galactic latitude (|b| < 15 degr) over 4,800 square degrees in the DENIS database. We used late-M (>=M8.0), L, and T dwarfs with accurate trigonometric parallaxes to calibrate the M_J versus I-J colour-luminosity relation. The resulting photometric distances have standard errors of ~15%, which we used to select candidates d_phot <= 30 pc. We measured proper motions from multi-epoch images found in the public archives ALADIN, DSS, 2MASS, DENIS, with at least three distinct epochs and time baselines of 10 to 21 years. We then used a Maximum Reduced Proper Motion cutoff to select 28 candidates as ultracool dwarfs (M8.0--L8.0) and to reject one as a distant red star. No T dwarf candidates were found in this search which required an object to be detected in all three DENIS bands. Our low-resolution optical spectra confirmed that 26 of them were indeed ultracool dwarfs, with spectral types from M8.0 to L5.5. Two contaminants and one rejected by the Maximum Reduced Proper Motion cutoff were all reddened F-K main sequence stars. 20 of these 26 ultracool dwarfs are new nearby ultracool dwarf members, three L dwarfs within 15 pc with one L3.5 at only ~10 pc. We determine a stellar density of \barΦ_J cor=(1.64 +- 0.46).10^{-3} dwarfs pc^{-3} mag^{-1} over 11.1 <= M_J <= 13.1 based on that sample of M8--L3.5 ultracool dwarfs. Our ultracool dwarf density value is in good agreement with the Cruz et al. measurement of the ultracool dwarf density at high Galactic latitude.

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arXiv2007

Simultaneous Multi-Wavelength Observations of Magnetic Activity in Ultracool Dwarfs. I. The Complex Behavior of the M8.5 Dwarf TVLM513-46546

[Abridged] We present the first simultaneous radio, X-ray, ultraviolet, and optical spectroscopic observations of the M8.5 dwarf TVLM513-46546, with a duration of 9 hours. These observations are part of a program to study the origin of magnetic activity in ultracool dwarfs, and its impact on chromospheric and coronal emission. Here we detect steady quiescent radio emission superposed with multiple short-duration, highly polarized flares; there is no evidence for periodic bursts previously reported for this object, indicating their transient nature. We also detect soft X-ray emission, with L_X/L_bol~10^-4.9, the faintest to date for any object later than M5, and a possible weak X-ray flare. TVLM513-46546 continues the trend of severe violation of the radio/X-ray correlation in ultracool dwarfs, by nearly 4 orders of magnitude. From the optical spectroscopy we find that the Balmer line luminosity exceeds the X-ray luminosity by a factor of a few, suggesting that, unlike in early M dwarfs, chromospheric heating may not be due to coronal X-ray emission. More importantly, we detect a sinusoidal H-alpha light curve with a period of 2 hr, matching the rotation period of TVLM513-46546. This is the first known example of such Balmer line behavior, which points to a co-rotating chromospheric hot spot or an extended magnetic structure, with a covering fraction of about 50%. This feature may be transitory based on the apparent decline in light curve peak during the four observed maxima. From the radio data we infer a large scale steady magnetic field of ~100 G, in good agreement with the value required for confinement of the X-ray emitting plasma. The radio flares, on the other hand, are produced in a component of the field with a strength of ~3 kG and a likely multi-polar configuration.

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arXiv2006

Discovery of Radio Emission from the Tight M8 Binary: LP 349-25

We present radio observations of 8 ultracool dwarfs with a narrow spectral type range (M8-M9.5) using the Very Large Array at 8.5 GHz. Only the tight M8 binary LP 349-25 was detected. LP 349-25 is the tenth ultracool dwarf system detected in radio and its trigonometric parallax pi = 67.6 mas, recently measured by Gatewood et al., makes it the furthest ultracool system detected by the Very Large Array to date, and the most radio-luminous outside of obvious flaring activity or variability. With a separation of only 1.8 AU, masses of the components of LP 349-25 can be measured precisely without any theoretical assumptions (Forveille et al.), allowing us to clarify their fully-convective status and hence the kind of magnetic dynamo in these components which may play an important role to explain our detection of radio emission from these objects. This also makes LP 349-25 an excellent target for further studies with better constraints on the correlations between X-ray, radio emission and stellar parameters such as mass, age, temperature, and luminosity in ultracool dwarfs.

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arXiv2006

Magnetic fields in M dwarfs: rapid magnetic field variability in EV Lac

We report here our spectropolarimetric observations obtained using the Espadons/CFHT high resolution spectrograph of two M dwarf stars which standard models suggest are fully convective: EV Lac (M3.5) and HH And (M5.5). The difference in their rotational velocity makes them good targets to study the dependence of the magnetic field topology in M dwarfs on rotation. Our results reveal some aspects of the field topology in EV Lac and HH And. We measured mean longitudinal magnetic field strengths (Bz) in EV Lac ranging from 18+-3 G to -40+-3 G. The Bz variations are seen to occur in a timescale of less than 50 minutes, significantly shorter than the rotation period, and are not due to a flaring event. We discuss some formation scenarios of the Zeeman signatures found in EV Lac. For HH And we could not detect circular polarization and thus we place an upper limit to Bz of 5 G.

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arXiv2006

LP 714-37: A wide pair of ultracool dwarfs actually is a triple

LP 714-37 was identified by Phan-Bao et al. (2005) as one of the very few wide pairs of very low mass (VLM) stars known to date, with a separation of 33 AU. Here we present adaptive optics imaging which resolves the secondary of the wide pair into a tighter binary, with a projected angular separation of 0.36 arcsec, or 7 AU. The estimated spectral types of LP 714-37B and LP 714-37C are M8.0 and M8.5. We discuss the implications of this finding for brown dwarf formation scenarios.

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arXiv2005

Discovery of a nearby M9 dwarf

We report the discovery of a new M9.0 dwarf at only 8.2 pc, which we identified in our search for nearby ultracool dwarf (I-J >= 3.0, later than M8.0) in the DENIS database. We measure a very high proper motion of 2.5 arc-sec/yr. The PC3 index measured from its low-resolution spectrum gives a spectrophotometric distance of 8.2 pc. This makes it the third closest M9.0 dwarf.

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arXiv2005

Spectroscopic distances of nearby ultracool dwarfs

We present updated results of spectroscopic follow-up observations of a sample of 45 M dwarf candidates identified by Phan-Bao et al. (2003) based on the DENIS photometry and proper motion measurements. Forty one of these are nearby late-M dwarfs (d < 30 pc) with spectral types ranging from M5.0 to M8.5 computed from the spectral indices. One contaminant is probably an F-G main sequence star reddened by intervening dust and three stars that were not observed have previous classifications as M dwarfs in the literature. In this paper, we identify three M7.5, five M8.0, one M8.5 dwarf and confirm two new M8.0 dwarf members of the 25 pc volume.

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arXiv2005

Discovery of a widely separated binary system of very low mass stars

We report our discovery of a nearby wide binary system, LP 714-37 AB. Phan-Bao et al. identified LP 714-37 as a mid-M dwarf during a cross-identification of the NLTT and DENIS catalogues. Our CCD images resolve the system into a binary with a projected angular separation of 1.8 arcsec, or 33 AU, and low-resolution optical spectra give spectral types of M5.5 (LP 714-37A) and M7.5 (LP 714-37B). This makes LP 714-37 AB one of very few widely separated (separation > 30 AU) very low-mass binary systems known in the field, and we discuss it in the context of the multiplicity properties of very low-mass stars and brown dwarfs.

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arXiv2005

New neighbours VI. Spectroscopy of DENIS nearby stars candidates

We present spectra of 36 nearby star candidates and 3 red giant candidates, identified in the DENIS database by Phan-Bao et al. (2003). 32 of the dwarf candidates are indeed nearby red dwarfs, with spectral types from M5.5 to M8.5. Out of 11 targets with low proper motion (mu < 0.1$ arc-sec/yr) but a Reduced Proper Motion above an inclusive threshold, 9 are red dwarfs. The 4 contaminants are all reddened F-K main sequence stars, and they could have been eliminated by checking for some well known high latitude molecular clouds. These stars might be of interest as probes of interstellar absorption. For the red dwarfs we derive spectral types and spectroscopic distances, using a new calibration of the PC3 spectral index to absolute magnitudes in the I, J, H and K photometric bands. We confirm 2 new members of the 12 pc volume (2 new M8.5), and one M7.5 NLTT object closer than 10pc. We also show that one quarter of the stars with photometric distances under 30 pc have too small a proper motion for inclusion in the NLTT catalog.

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arXiv2003

New neighbours: V. 35 DENIS late-M dwarfs between 10 and 30 parsecs

This paper reports updated results on our systematic mining of the DENIS database for nearby very cool M-dwarfs (M6V-M8V, 2.0<I-J<3.0, photometric distance within 30 pc), We calibrate the DENIS (M_I, I-J) colour-luminosity relationship from M dwarfs with well measured parallaxes (HIP, GCTP,...), obtaining distance errors for single dwarfs of 25%. Using proper motions measured on archive Schmidt plates for stars that meet the photometric selection criteria, we eliminate the giants by a Reduced Proper Motion cutoff, which is significantly more selective than a simple proper motion cutoff. Here we present new data for 62 red dwarf candidates selected over 5700 square degrees in the DENIS database. 26 of those originate in the 2100 square degrees analysed in Paper I, with improved parameters here, and 36 were found in 3600 additional square degrees. 25 of those are new nearby dwarfs. We determine from that sample of 62 stars a stellar density for 12.0<M_I<14.0 of 2.2(+-0.4)E-3 stars/pc^3/mag. This value is consistent with photometric luminosity functions measured from deeper and smaller-field observations, but not with the nearby star luminosity function. In addition we cross-identified the NLTT and DENIS catalogues to find 15 similar stars, in parts of the sky not yet covered by the colour-selected search.

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arXiv2001

New neighbours: IV. 30 DENIS late-M dwarfs between 15 and 30 parsecs

We present 30 new nearby (d<30 pc) red dwarf candidates, with estimated spectral types M6 to M8. 26 were directly selected from the DENIS database, and another 4 were first extracted from the LHS catalogue and cross-identified with a DENIS counterpart. Their proper motions were measured on the MAMA measuring machine from plates spanning 13 to 48 years, and are larger than 0.1 arcsec/yr, ruling out the possibility that they are giants. Their distances were estimated from the DENIS colours and IR colour-magnitude relations and range between 15 and 30 pc.

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