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Register a new userSubtle Signatures of Multiplicity in Late-type Dwarf Spectra: The Unresolved M8.5 + T5 Binary 2MASS J03202839-0446358
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Adam J. Burgasser
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Evidence is presented that 2MASS J03202839-0446358, a late-type dwarf with discrepant optical (M8:) and near-infrared (L1) spectral types, is an as-yet unresolved stellar/brown dwarf binary with late-type M dwarf and T dwarf components. This conclusion is based on low-resolution, near-infrared spectroscopy that reveals a subtle but distinctive absorption feature at 1.6 micron. The feature, which is also present in the combined light spectrum of the M8.5 + T6 binary SCR 1845-6357, arises from the combination of FeH absorption from an M8.5 primary and pseudo-continuum flux from a T5+/-1 secondary, as ascertained from binary spectral templates constructed from empirical data. The binary templates provide a far superior match to the overall near-infrared spectral energy distribution of 2MASS J0320-0446 than any single comparison spectra. Laser guide star adaptive optics (LGS AO) imaging observations, including the first application of LGS AO aperture mask interferometry, fail to resolve a faint companion, restricting the projected separation of the system to less than 8.3 AU at the time of observation. 2MASS J0320-0446 is the second very low mass binary to be identified from unresolved, low-resolution, near-infrared spectroscopy, a technique that complements traditional high resolution imaging and spectroscopic methods.
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[not part of RNAAS note] We report the discovery of WISE J000100.45+065259.6 as a very low mass star/brown dwarf spectral binary candidate, on the basis of low resolution near-infrared spectroscopy obtained with IRTF/SpeX. Decomposition of the spectrum indicates component types of M8.5+T5 with a predicted $Delta{J}$ = 3.5. As the majority of confirmed spectral binary candidates to date are very closely-separated systems ($rho$ $lesssim$ 3 AU; $P$ $lesssim$ 15~yr), this source may provide mass measurements across the hydrogen burning limit within the decade.
The observable spectrum of an unresolved binary star system is a superposition of two single-star spectra. Even without a detectable velocity offset between the two stellar components, the combined spectrum of a binary system is in general different from that of either component, and fitting it with single-star models may yield inaccurate stellar parameters and abundances. We perform simple experiments with synthetic spectra to investigate the effect of unresolved main-sequence binaries on spectral fitting, modeling spectra similar to those collected by the APOGEE, GALAH, and LAMOST surveys. We find that fitting unresolved binaries with single-star models introduces systematic biases in the derived stellar parameters and abundances that are modest but certainly not negligible, with typical systematic errors of $300,rm K$ in $T_{rm eff}$, 0.1 dex in $log g$, and 0.1 dex in $[rm Fe/H]$ for APOGEE-like spectra of solar-type stars. These biases are smaller for spectra at optical wavelengths than in the near-infrared. We show that biases can be corrected by fitting spectra with a binary model, which adds only two labels to the fit and includes single-star models as a special case. Our model provides a promising new method to constrain the Galactic binary population, including systems with single-epoch spectra and no detectable velocity offset between the two stars.
Low-mass and brown dwarfs have recently been found as wide companions to many nearby stars, formerly believed to be single. Wide binaries are usually found as common proper motion pairs. Sometimes, more than two objects share the same large proper motion, identifying them as nearby systems. We have found a third, low-mass component to a known wide binary at a distance of about 21pc, consisting of a red and a white dwarf (LHS4039 and LHS4040; about 150AU separation). The new companion, APMPMJ2354-3316C separated by about 2200AU, was classified as M8.5 dwarf. In recent spectroscopic observations it shows a very strong $H_{alpha}$ emission line and blue continuum. Comparing this event to flares in late-type M dwarfs, we find some similarity with LHS2397a, a nearby M8 dwarf which is so far the only known example of a low-mass star with a tight brown dwarf companion (separation of less than 4AU). The level of the activity as measured by $L_{H_{alpha}}/L_{bol}$ is comparable to that of the M9.5 dwarf 2MASSWJ0149+29 both during the flare and in quiescence.
We examine the consequences of, and apply, the formalism developed in Terquem (2021) for calculating the rate $D_R$ at which energy is exchanged between fast tides and convection. In this previous work, $D_R$ (which is proportional to the gradient of the convective velocity) was assumed to be positive in order to dissipate the tidal energy. Here we argue that, even if energy is intermittently transferred from convection to the tides, it must ultimately return to the convective flow and transported efficiently to the stellar surface on the convective timescale. This is consistent with, but much less restrictive than, enforcing $D_R>0$. Our principle result is a calculation of the circularization timescale of late-type binaries, taking into account the full time evolution of the stellar structure. We find that circularization is very efficient during the PMS phase, inefficient during the MS, and once again efficient when the star approaches the RGB. These results are in much better agreement with observations than earlier theories. We also apply our formalism to hot Jupiters, and find that tidal dissipation in a Jupiter mass planet yields a circularization timescale of 1 Gyr for an orbital period of 3 d, also in good overall agreement with observations. The approach here is novel, and the apparent success of the theory in resolving longstanding timescale puzzles is compelling.
Element diffusion is expected to occur in all kinds of stars : according to the relative effect of gravitation and radiative acceleration, they can fall or be pushed up in the atmospheres. Helium sinks in all cases, thereby creating a gradient at the bottom of the convective zones. This can have important consequences for the sound velocity, as has been proved in the sun with helioseismology. We investigate signatures of helium diffusion in late F-type stars by asteroseismology. Stellar models were computed with different physical inputs (with or without element diffusion) and iterated in order to fit close-by evolutionary tracks for each mass. The theoretical oscillation frequencies were computed and compared for pairs of models along the tracks. Various asteroseismic tests (large separations, small separations, second differences) were used and studied for the comparisons. The results show that element diffusion leads to changes in the frequencies for masses larger than 1.2 Msun. In particular the helium gradient below the convective zone should be detectable through the second differences.
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