Optical Spectroscopy of a Flare on Barnard’s Star

The nearby red dwarf binary GJ65 AB (UV+BL Ceti, M5.5Ve+M6Ve) is a cornerstone system to probe the physics of very low-mass stars. The radii of the two stars are currently known only from indirect photometric estimates, however, and this prevents us from using GJ65 AB as calibrators for the mass-radius (M-R) relation. We present new interferometric measurements of the angular diameters of the two components of GJ65 with the VLTI/PIONIER instrument in the near-infrared H band: θ UD (A) = 0.558 ± 0.008 ± 0.020 mas and θ UD (B) = 0.539 ± 0.009 ± 0.020 mas. They translate into limb-darkened angular diameters of θ LD (A) = 0.573 ± 0.021 mas and θ LD (B) = 0.554 ± 0.022 mas. Based on the known parallax, the linear radii are R(A) = 0.165 ± 0.006 R and R(B) = 0.159 ± 0.006 R (σ(R)/R = 4%). We searched for the signature of flares and faint companions in the interferometric visibilities and closure phases, but we did not identify any significant signal. We also observed GJ65 with the VLT/NACO adaptive optics and refined the orbital parameters and infrared magnitudes of the system. We derived masses for the two components of m(A) = 0.1225 ± 0.0043 M and m(B) = 0.1195 ± 0.0043 M (σ(m)/m = 4%). To derive the radial and rotational velocities of the two stars as well as their relative metallicity with respect to Proxima, we also present new individual UVES high-resolution spectra of the two components. Placing GJ65 A and B in the mass-radius diagram shows that their radii exceed expectations from recent models by 14 ± 4% and 12 ± 4% , respectively. Following previous theories, we propose that this discrepancy is caused by the inhibition of convective energy transport by a strong internal magnetic field generated by dynamo effect in these two fast-rotating stars. A comparison with the almost identical twin Proxima, which is rotating slowly, strengthens this hypothesis because the radius of Proxima does not appear to be inflated compared to models.

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“…4.5). Similar emission lines were observed during a flare of the M4V dwarf GJ699 by Paulson et al (2006).…”

Section: Ut Datesupporting

confidence: 76%

The red dwarf pair GJ65 AB: inflated, spinning twins of Proxima

Kervella

Mérand

Ledoux

et al. 2016

A&A

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“…For even higher members of the Balmer series, it is not clear whether their broad line components start to merge. Moreover, Paulson et al (2006) also found less broadening in higher Balmer lines and gave NLTE effects as a possible reason, which affect low order lines most.…”

Section: Stark Broadening Versus Turbulent Broadeningmentioning

confidence: 87%

“…They can be blue-or (more often) red-shifted, and are often ascribed to a moving turbulent plasma component. Broadening, especially of hydrogen lines, may also be caused by the Stark effect; for a flare on Barnard's star, Paulson et al (2006) attributed a symmetric line broadening to the Stark effect. Also, for the Sun Johns- Krull et al (1997) found evidence of Stark broadening in higher order Balmer lines during a strong flare.…”

Section: Broad Line Componentsmentioning

confidence: 99%

A multi-wavelength view of AB Doradus outer atmosphere

Lalitha

Fuhrmeister

Wolter

et al. 2013

A&A

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Aims. We study the chromosphere and corona of the ultra-fast rotator AB Dor A at high temporal and spectral resolution using simultaneous observations with XMM-Newton in the X-rays, VLT/UVES in the optical, and the ATCA in the radio. Our optical spectra have a resolving power of ∼50 000 with a time cadence of ∼1 min. Our observations continuously cover more than one rotational period and include both quiescent periods and three flaring events of different strengths. Methods. From the X-ray observations we investigated the variations in coronal temperature, emission measure, densities, and abundance. We interpreted our data in terms of a loop model. From the optical data we characterised the flaring chromospheric material using numerous emission lines that appear in the course of the flares. A detailed analysis of the line shapes and line centres allowed us to infer physical characteristics of the flaring chromosphere and to coarsely localise the flare event on the star. Results. We specifically used the optical high-cadence spectra to demonstrate that both turbulent and Stark broadening are present during the first ten minutes of the first flare. Also, in the first few minutes of this flare, we find short-lived (one to several minutes) emission subcomponents in the Hα and Ca ii K lines, which we interpret as flare-connected shocks owing to their high intrinsic velocities. Combining the space-based data with the results of our optical spectroscopy, we derive flare-filling factors. Finally, comparing X-ray, optical broadband, and line emission, we find a correlation for two of the three flaring events, while there is no clear correlation for one event. Also, we do not find any correlation of the radio data to any other observed data.

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“…Stark broadening is thought to be an important symmetric broadening mechanism of the hydrogen Balmer lines during solar and stellar flares (Svestka, 1963;Worden et al, 1984;Hawley and Pettersen, 1991;Johns-Krull et al, 1997;Paulson et al, 2006;Allred et al, 2006), in addition to possible turbulent (Eason et al, 1992;Doyle et al, 1988) and thermal contributions. The linear (first order) Stark effect is the splitting of the degenerate orbital angular momentum (l) states of each principal quantum state (n) of hydrogen, due to a net electric microfield from the surrounding distribution of charges.…”

Section: Stark Broadening At the Balmer Edgementioning

confidence: 99%

New Insights into White-Light Flare Emission from Radiative-Hydrodynamic Modeling of a Chromospheric Condensation

et al. 2015

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The heating mechanism at high densities during M-dwarf flares is poorly understood. Spectra of M-dwarf flares in the optical and near-ultraviolet wavelength regimes have revealed three continuum components during the impulsive phase: 1) an energetically dominant blackbody component with a color temperature of T ≈ 10 4 K in the blue-optical, 2) a smaller amount of Balmer continuum emission in the near-ultraviolet at λ ≤ 3646Å and 3) an apparent pseudo-continuum of blended high-order Balmer lines between λ = 3646Å and λ ≈ 3900Å. These properties are not reproduced by models that employ a typical solar-type flare heating level of ≤ 10 11 erg cm −2 s −1 in non-thermal electrons, and therefore our understanding of these spectra is limited to a phenomenological three-component interpretation. We present a new 1D radiative-hydrodynamic model of an M-dwarf flare from precipitating non-thermal electrons with a large energy flux of 10 13 erg cm −2 s −1 . The simulation produces bright nearultraviolet and optical continuum emission from a dense (n >10 15 cm −3 ), hot (T ≈ 12 000 − 13 500 K) chromospheric condensation. For the first time, the observed color temperature and Balmer jump ratio are produced self-consistently in a radiative-hydrodynamic flare model. We find that a T ≈ 10 4 K blackbodylike continuum component and a small Balmer jump ratio result from optically thick Balmer (∞ → n = 2) and Paschen recombination (∞ → n = 3) radiation, and thus the properties of the flux spectrum are caused by blue (λ ≈ 4300Å) light escaping over a larger physical depth range compared to red (λ ≈ 6700Å) and near-ultraviolet (λ ≈ 3500Å) light. To model the near-ultraviolet pseudocontinuum previously attributed to overlapping Balmer lines, we include the extra Balmer continuum opacity from Landau-Zener transitions that result from merged, high order energy levels of hydrogen in a dense, partially ionized atmosphere. This reveals a new diagnostic of ambient charge density in the densest regions of the atmosphere that are heated during dMe and solar flares.

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Optical Spectroscopy of a Flare on Barnard’s Star

Cited by 48 publications

References 42 publications

The red dwarf pair GJ65 AB: inflated, spinning twins of Proxima

The red dwarf pair GJ65 AB: inflated, spinning twins of Proxima

A multi-wavelength view of AB Doradus outer atmosphere

New Insights into White-Light Flare Emission from Radiative-Hydrodynamic Modeling of a Chromospheric Condensation

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