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Register a new userPulsations induced by deuterium-burning in young brown dwarfs
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Very low-mass stars and brown dwarfs can undergo pulsational instability excited by central deuterium burning during the initial phases of their evolution. We present the results of evolutionary and nonadiabatic linear stability models that show the presence of unstable fundamental modes. The pulsation periods vary bewteen ~5 hr for a 0.1 Msun star and ~1 hr for a 0.02 M$_odot$ brown dwarf. The results are rather insensitive to variations in the input physics of the models. We show the location of the instability strip in the HR and c-m diagrams and discuss the observational searches for young pulsators in nearby star forming regions.
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There is no universally acknowledged criterion to distinguish brown dwarfs from planets. Numerous studies have used or suggested a definition based on an objects mass, taking the ~13-Jupiter mass (M_J) limit for the ignition of deuterium. Here, we investigate various deuterium-burning masses for a range of models. We find that, while 13 M_J is generally a reasonable rule of thumb, the deuterium fusion mass depends on the helium abundance, the initial deuterium abundance, the metallicity of the model, and on what fraction of an objects initial deuterium abundance must combust in order for the object to qualify as having burned deuterium. Even though, for most proto-brown dwarf conditions, 50% of the initial deuterium will burn if the objects mass is ~(13.0 +/- 0.8)M_J, the full range of possibilities is significantly broader. For models ranging from zero-metallicity to more than three times solar metallicity, the deuterium burning mass ranges from ~11.0 M_J (for 3-times solar metallicity, 10% of initial deuterium burned) to ~16.3 M_J (for zero metallicity, 90% of initial deuterium burned).
In the absence of a third dredge-up episode during the asymptotic giant branch phase, white dwarf models evolved from low-metallicity progenitors have a thick hydrogen envelope, which makes hydrogen shell burning be the most important energy source. We investigate the pulsational stability of white dwarf models with thick envelopes to see whether nonradial $g$-mode pulsations are triggered by hydrogen burning, with the aim of placing constraints on hydrogen shell burning in cool white dwarfs and on a third dredge-up during the asymptotic giant branch evolution of their progenitor stars. We construct white-dwarf sequences from low-metallicity progenitors by means of full evolutionary calculations, and analyze their pulsation stability for the models in the range of effective temperatures $T_{rm eff} sim 15,000,-, 8,000$ K. We demonstrate that, for white dwarf models with masses $M_{star} lesssim 0.71,rm M_{sun}$ and effective temperatures $8,500 lesssim T_{rm eff} lesssim 11,600$ K that evolved from low-metallicity progenitors ($Z= 0.0001$, $0.0005$, and $0.001$) the dipole ($ell= 1$) and quadrupole ($ell=2$) $g_1$ modes are excited mostly due to the hydrogen-burning shell through the $varepsilon$-mechanism, in addition to other $g$ modes driven by either the $kappa-gamma$ or the convective driving mechanism. However, the $varepsilon$ mechanism is insufficient to drive these modes in white dwarfs evolved from solar-metallicity progenitors. We suggest that efforts should be made to observe the dipole $g_1$ mode in white dwarfs associated with low-metallicity environments, such as globular clusters and/or the galactic halo, to place constraints on hydrogen shell burning in cool white dwarfs and the third dredge-up episode during the preceding asymptotic giant branch phase.
We present the results of a nonadiabatic, linear stability analysis of models of very low-mass stars (VLMSs) and brown dwarfs (BDs) during the deuterium burning phase in the center. We find unstable fundamental modes with periods varying between ~5 hr for a 0.1 Msun star and ~1 hr for a 0.02 Msun BD. The growth time of the instability decreases with decreasing mass and remains well below the deuterium burning time scale in the mass range considered (0.1--0.02 Msun). These results are robust against variations of the relevant input physics in the evolutionary models. We identify possible candidates for pulsational variability among known VLMSs and BDs in nearby star forming regions whose location in the HR diagram falls within or close to the boundary of the instability strip. Finally, we discuss the possibility that the variability observed in a few objects with periods of ~1 hr can be interpreted in terms of pulsation.
The protostellar outflow mechanism operates for a significant fraction of the pre-main sequence phase of a solar mass star and is thought to have a key role in star and perhaps even planet formation. This energetic mechanism manifests itself in several different forms and on many scales. Thus outflow activity can be probed in numerous different regimes from radio to X-ray wavelengths. Recent discoveries have shown that it is not only solar mass stars that launch outflows during their formation but also the sub-stellar brown dwarfs. In this article what is currently known about jets from young stars is summarised, including an outline of why it is important to study jets. The second part of this article is dedicated to jets from young brown dwarfs. While only a small number of brown dwarf outflows have been investigated to date, interesting properties have been observed. Here observations of brown dwarf outflows are described and what is currently known of their properties compared to low mass protostellar outflows.
We suggest that a high proportion of brown dwarfs are formed by gravitational fragmentation of massive extended discs around Sun-like stars. Such discs should arise frequently, but should be observed infrequently, precisely because they fragment rapidly. By performing an ensemble of radiation-hydrodynamic simulations, we show that such discs fragment within a few thousand years, and produce mainlybrown dwarf (BDs) stars, but also planetary mass (PM) stars and very low-mass hydrogen-burning (HB) stars. Most of the the PM stars and BDs are ejected by mutual interactions. We analyse the statistical properties of these stars, and compare them with observations. After a few hundred thousand years the Sun-like primary is typically left with a close low-mass HB companion, and two much wider companions: a low-mass HB star and a BD star, or a BD-BD binary. There is a BD desert extending out to at least ~100 AU; this is because BDs tend to be formed further out than low-mass HB stars, and then they tend to be scattered even further out, or even into the field. BDs form with discs of a few Mj and radii of a few tens of AU, and they are more likely to retain these discs if they remain bound to the primary star. Binaries form by pairing of the newly-formed stars in the disc, giving a low-mass binary fraction of ~0.16. These binaries include close and wide BD/BD binaries and BD/PM binaries. BDs that remain as companions to Sun-like stars are more likely to be in BD/BD binaries than are BDs ejected into the field. Disc fragmentation is a robust mechanism; even if only a small fraction of Sun-like stars host the required massive extended discs,this mechanism can produce all the PM stars observed, most of the BD stars, and a significant proportion of the very low-mass HB stars.
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