Satoshi Honda

Abstract We conducted the time-resolved simultaneous optical spectroscopic and photometric observations of mid-M-dwarf flare stars YZ CMi, EV Lac, and AD Leo. Spectroscopic observations were obtained using Apache Point Observatory 3.5 m and Small and Moderate Aperture Research Telescope System 1.5 m telescopes during 31 nights. Among the 41 detected flares, seven flares showed clear blue wing asymmetries in the Hα line, with various correspondences in flare properties. The duration of the blue wing asymmetries range from 20 minutes to 2.5 hr, including a flare showing the shift from blue to red wing asymmetry. Blue wing asymmetries can be observed during both white-light and candidate non-white-light flares. All of the seven flares showed blue wing asymmetries also in the Hβ line, but there are large varieties on which other chromospheric lines showed blue wing asymmetries. One among the 7 flares was also observed with soft X-ray spectroscopy, which enabled us to estimate the flare magnetic field and length of the flare loop. The line-of-sight velocities of the blueshifted components range from –73 to –122 km s⁻¹. Assuming that the blueshifts were caused by prominence eruptions, the mass of upward-moving plasma was estimated to be 10¹⁵–10¹⁹ g, which are roughly on the relation between flare energy and erupting mass expected from solar coronal mass ejections (CMEs). Although further investigations are necessary for understanding the observed various properties, these possible prominence eruptions on M-dwarfs could evolve into CMEs, assuming the similar acceleration mechanism with solar eruptions.

Abstract We report on the chemical composition of the very metal-poor (VMP; [Fe/H] = −2.9) star LAMOST J1645+4357, which was identified as a red giant having peculiar abundance ratios by Li et al. The standard abundance analysis is carried out for this object and the well-studied metal-poor star HD 122563, which has similar atmospheric parameters. LAMOST J1645+4357 has a remarkable abundance set, highlighted by these features: (1) nitrogen is significantly enhanced ([N/Fe] = +1.4), and the total abundance of C and N is also very high ([(C+N)/Fe] = +0.9); (2) α-elements are overabundant with respect to iron, as generally found in VMP stars; (3) Ti, Sc, Co, and Zn are significantly deficient; (4) Cr and Mn are enhanced compared to most of VMP stars; and (5) Sr and Ba are deficient, and the Sr/Ba ratio ([Sr/Ba] = −1.0) is significantly lower than the value expected for the r-process. The overall abundance pattern of this object from C to Zn is well reproduced by a faint supernova model assuming spherical explosion, except for the excess of Cr and Mn, which requires enhancement of incomplete Si burning or small contributions from a Type Ia or pair-instability supernova. There remains, however, a question as to why the abundance pattern of this star is so unique among VMP stars.

Abstract Starspots and stellar flares are indicators of stellar magnetic activity. The magnetic energy stored around spots is thought to be the origin of flares, but the connection is not completely understood. To investigate the relation between spot locations deduced from light curves and the occurrence of flares therein, we perform starspot modeling for the TESS light curves of three M-dwarf flare stars, AU Mic, YZ CMi, and EV Lac, using the code implemented in Paper I. The code enables us to deduce multiple stellar/spot parameters by the adaptive parallel tempering algorithm efficiently. We find that flare occurrence frequency is not necessarily correlated with the rotation phases of the light curve for each star. The result of starspot modeling shows that any spot is always visible to the line of sight in all phases, and we suggest that this can be one of the reasons why there is no or low correlation between rotation phases and flare frequency. In addition, the amplitude and shape of the light curve for AU Mic and YZ CMi have varied in two years between different TESS cycles. The result of starspot modeling suggests that this can be explained by the variations of spot size and latitude.

Abstract Stellar coronal mass ejections (CMEs) have recently received much attention for their impacts on exoplanets and stellar evolution. Detecting prominence eruptions, the initial phase of CMEs, as the blueshifted excess component of Balmer lines is a technique to capture stellar CMEs. However, most of prominence eruptions identified thus far have been slow and less than the surface escape velocity. Therefore, whether these eruptions were developing into CMEs remained unknown. In this study, we conducted simultaneous optical photometric observations with Transiting Exoplanet Survey Satellite and optical spectroscopic observations with the 3.8 m Seimei Telescope for the RS CVn-type star V1355 Orionis that frequently produces large-scale superflares. We detected a superflare releasing 7.0 × 10³⁵ erg. In the early stage of this flare, a blueshifted excess component of Hα extending its velocity up to 760–1690 km s⁻¹ was observed and thought to originate from prominence eruptions. The velocity greatly exceeds the escape velocity (i.e., ∼350 km s⁻¹), which provides important evidence that stellar prominence eruptions can develop into CMEs. Furthermore, we found that the prominence is very massive (9.5 × 10¹⁸ g < M < 1.4 × 10²¹ g). These data will clarify whether such events follow existing theories and scaling laws on solar flares and CMEs even when the energy scale far exceeds solar cases.

<jats:title>Abstract</jats:title> <jats:p>Active M-type stars are known to often produce superflares on the surface. Radiation from stellar (super)flares is important for exoplanet habitability, but the mechanisms are not well understood. In this paper, we report simultaneous optical spectroscopic and photometric observations of a stellar superflare on an active M dwarf, YZ Canis Minoris, with the 3.8 m Seimei telescope and the Transiting Exoplanet Survey Satellite. The flare bolometric energy is <jats:inline-formula> <jats:tex-math> <?CDATA ${1.3}_{-0.6}^{+1.6}\times {10}^{34}\,\mathrm{erg}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>1.3</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.6</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>1.6</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>×</mml:mo> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>34</mml:mn> </mml:mrow> </mml:msup> <mml:mspace width="0.25em" /> <mml:mi>erg</mml:mi> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjacb928ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> and the H<jats:italic>α</jats:italic> energy is <jats:inline-formula> <jats:tex-math> <?CDATA ${3.0}_{-0.1}^{+0.1}\times {10}^{32}\,\mathrm{erg}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>3.0</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.1</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.1</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>×</mml:mo> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>32</mml:mn> </mml:mrow> </mml:msup> <mml:mspace width="0.25em" /> <mml:mi>erg</mml:mi> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjacb928ieqn2.gif" xlink:type="simple" /> </jats:inline-formula>. The H<jats:italic>α</jats:italic> emission line profile shows red asymmetry throughout the flare, with a duration of 4.6–5.1 hr. The velocity of the red asymmetry is ∼200–500 km s<jats:sup>–1</jats:sup> and the line width of H<jats:italic>α</jats:italic> broadens up to 34 ± 14 Å. The redshifted velocity and line width of H<jats:italic>α</jats:italic> line decay more rapidly than the equivalent width, and their time evolutions are correlated with that of the white-light emission. This indicates the possibility of the white light, the H<jats:italic>α</jats:italic> red asymmetry, and the H<jats:italic>α</jats:italic> line broadening originating from nearly the same site, i.e., the dense chromospheric condensation region, heated by nonthermal electrons. On the other hand, the flux ratio of the redshifted excess components to the central components is enhanced one hr after the flare’s onset. This may be due to the main source of the red asymmetry changing to post-flare loops in the later phase of the flare.</jats:p>