arXiv subjects

Mai Yamashita

Publications and source records attributed to Mai Yamashita.

8 recordsLinked to original sources

arXiv2026

Magnetic Heating Across the Sun and Solar-like Stars: Universal Scaling Laws from Chromospheres to Coronae

Magnetic activity in cool stars governs the thermal structure and high-energy radiative output of their outer atmospheres, thereby influencing stellar evolution, stellar winds, and the atmospheres of orbiting (exo)planets. A long-standing question in stellar astrophysics is whether the mechanisms responsible for atmospheric heating are universal across the Sun and cool stars spanning different ages and activity levels. In this paper, we review recent progress in understanding magnetic heating across the Sun and Sun-like stars through empirical scaling relations between photospheric magnetic flux and radiative output from the chromosphere, transition region, and corona. Analysis of more than a decade of Sun-as-a-star observations reveals that irradiance and magnetic flux follow power-law relationships over a wide temperature range. While coronal emissions exhibit superlinear scaling with magnetic flux, chromospheric and transition-region diagnostics show weaker, sublinear dependencies. Remarkably, observations of G-type stars with ages ranging from 50 Myr to 4.5 Gyr are found to lie on extensions of the solar scaling laws, suggesting that a common magnetic-heating mechanism operates across different levels of stellar activity. We further discuss the application of these scaling relations to reconstructing stellar X-ray and ultraviolet (XUV) spectra from observed magnetic fluxes. The resulting synthetic spectra reproduce actual observations of young, active solar analogs, providing a practical tool to estimate the ionizing radiation whose extreme UV emissions cannot be directly measured.

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arXiv2026

A Systematic Study of Quiescent and Outburst Properties of X-ray-bright Young Stellar Objects Using XMM-Newton

Young Stellar Objects (YSOs) exhibit strong X-ray emission, widely attributed to magnetic reconnection and magnetospheric accretion; however, owing primarily to limited photon statistics, observational tests of these mechanisms have often relied on simplified analyses, leaving room for more precise constraints on their emission processes. We aim to derive the X-ray properties of X-ray-bright YSOs selected from multiple star clusters and investigate their emission mechanisms through an approach from multiple perspectives based on timing and spectroscopic analysis. We performed a systematic search of XMM-Newton archival observations and constructed a sample of 51 X-ray-bright YSOs for timing and spectroscopic analyses. We identified quiescent and outburst phases through timing analysis, performed phase-resolved multi-temperature spectroscopy. Multi-temperature plasma structures are detected in both quiescent and outburst emission. The relationships among the timing and spectral parameters are broadly consistent with a magnetic-reconnection scenario. Comparison with Gaia DR3 stellar parameters suggests that magnetospheric accretion alone has difficulty explaining most of the fitted X-ray temperatures, although the coolest model components with $kT \lesssim 0.3$ keV may include an accretion-shock contribution. After separating these low-temperature components, the remaining quiescent components show a clearer positive correlation in the $EM$--$kT$ plane, with magnetic-loop lengths distributed around $10^{12}$ cm. Neupert-like behavior, the temporal evolution of temperature and emission measure, the Rossby-number activity relation, and positive correlations between quiescent and outburst properties all suggest that magnetic reconnection plays a key role in both phases.

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arXiv2025

Variations in the magnetic field strength of pre-main-sequence stars, solar-type main-sequence stars, and the Sun

The surface magnetic fields of pre-main-sequence stars and zero-age main-sequence stars are notably strong, resulting in the generation of numerous spots and the emission of bright chromospheric lines. Rotational variations in magnetic field strength have been identified in T Tauri stars (TTSs) and young main-sequence stars using Zeeman--Doppler imaging. This study investigates the relationship between the mean values and variation amplitudes of the magnetic field strengths of TTSs, main-sequence stars, and the Sun. The findings reveal a positive correlation of over three orders of magnitude, suggesting that a common mechanism drives the magnetic fields of these stars. This positive correlation implies that stars with larger spot sizes experience greater variation amplitudes due to rotational modulations. For the Sun, both the mean magnetic field strength value and its variation amplitude tend to be higher during solar maximum than during solar minimum.

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arXiv2024

Chromospheric Mg I Emission Lines of Pre-Main-Sequence Stars

To reveal details of the internal structure, the relationship between chromospheric activity and the Rossby number has been extensively examined for main-sequence stars. For active pre-main sequence (PMS) stars, it is suggested that the level of activity be assessed using optically thin emission lines, such as Mg I. We aim to detect Mg I chromospheric emission lines from PMS stars and determine whether the chromosphere is activated by the dynamo process or by mass accretion from protoplanetary disks. We analyzed high-resolution optical spectra of $64\ $PMS stars obtained with Very Large Telescope (VLT)/X-shooter and UVES and examined the infrared Ca II (8542 A) and Mg I (8807 A) emission lines. To detect the weak chromospheric emission lines, we determined the atmospheric parameters ($T_{\rm eff}$ and $\log\ g$) and the degree of veiling of the PMS stars by comparing the observed spectra with photospheric model spectra. After subtracting the photospheric model spectrum from the PMS spectrum, we detected Ca II and Mg I as emission lines. The strengths of the Mg I emission lines in PMS stars with no veiling are comparable to those in zero-age main-sequence (ZAMS) stars if both types of stars have similar Rossby numbers. The Mg I emission lines in these PMS stars are thought to be formed by a dynamo process similar to that in ZAMS stars. In contrast, the Mg I emission lines in PMS stars with veiling are stronger than those in ZAMS stars. These objects are believed to have protoplanetary disks, where mass accretion generates shocks near the photosphere, heating the chromosphere. The chromosphere of PMS stars is activated not only by the dynamo process but also by mass accretion.

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arXiv2022

Starspots, chromospheric emission lines, and flares of zero-age main-sequence stars

Zero-age main-sequence (ZAMS) stars are considered to have enormous starspots and show strong chromospheric emission lines because of their strong surface magnetic field. We discuss the dynamo activities of ZAMS stars with respect to their periodic light variation caused by a starspot and with respect to the strength of the chromospheric emission lines. The light curves of $33$ ZAMS stars in IC 2391 and IC 2602 were obtained from \textit{TESS} photometric data. The light curves can be grouped into the following four categories: single frequency, possible shape changer, beater, and complex variability. The amplitudes of the light curves are $0.001-0.145\,\mathrm{mag}$, similar to those of ZAMS stars in Pleiades. The starspot coverages are $0.1-21\%$. We found that the light variations and Ca\,\emissiontype{II} emission line strength of ZAMS stars in IC 2391, IC 2602, and the Pleiades cluster are as large as those of the most active superflare stars and two orders larger than those of the Sun, and are located on the extensions of the superflare stars. These results suggest that superflare stars link the properties of the Sun to those of the ZAMS stars of ages between $30$ and $120\,\mathrm{Myr}$. ZAMS stars with a single frequency or possible shape change in the light curve tend to have both large light variation, indicating large spot coverage, and saturated Ca\,\emissiontype{II} emission line strength. ZAMS stars with beat or complex variability have small spot coverage and a faint Ca\,\emissiontype{II} emission line. We also detected $21$ flares in the \textit{TESS} light curves of $12$ ZAMS stars in IC 2391 and IC 2602, where most of these stars have saturated chromospheric Ca\,\emissiontype{II} emission lines. The energies of the flares are estimated to be $\sim 10^{33}-10^{35}\,\mathrm{erg}$, which is comparable with the energy of a superflare.

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arXiv2022

Measurements of Chromospheric Mg I Emission Lines of Zero-Age Main-Sequence Stars

The chromosphere is the active atmosphere in which energetic eruption events, such as flares, occur. Chromospheric activity is driven by the magnetic field generated by stellar rotation and convection. The relationship between chromospheric activity and the Rossby number, the ratio of the rotational period to the convective turnover time, has been extensively examined for many types of stars, by using narrow chromospheric emission lines, such as the Ca II lines and the Mg II h and k lines. However, the stars with small Rossby numbers, i.e., stars with rapid rotations and/or long convective turnover times, show constant strengths of such lines against the Rossby number. In this study, we investigate the infrared Mg I emission lines at 8807 A of 47 zero-age main-sequence (ZAMS) stars in IC 2391 and IC 2602 using the archive data of the Anglo-Australian Telescope at the University College London Echelle Spectrograph. After subtracting the photospheric absorption component, the Mg I line is detected as an emission line for 45 ZAMS stars, whose equivalent widths are between 0.02 A and 0.52 A. A total of 42 ZAMS stars show the narrower Mg I emission lines instead of the Ca II infrared triplet emission lines, suggesting that they are formed at different depths. The ZAMS stars with smaller Rossby numbers show stronger Mg I emission lines. The Mg I emission line is not saturated even in "the saturated regime of the Ca II emission lines," i.e., Rossby number < 10^(-1.1). The Mg I emission line is considered to be a good indicator of chromospheric activity, particularly for active objects.

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arXiv2021

Rotation-Activity Relation of Ca II and Mg I Infrared Emission Lines of Young Stars

To reveal the detail of the internal structure, the relationship between chromospheric activity and the Rossby number, N_R (= rotational period P / convective turnover time tau_c), has been extensively examined for main-sequence stars. The goal of our work is to apply the same methods to pre-main-sequence (PMS) stars and identify the appropriate model of tau_c for them. Yamashita et al. (2020) investigated the relationship between N_R and strengths of the Ca II infrared triplet (IRT; lambda 8498, 8542, 8662 A) emission lines of 60 PMS stars. Their equivalent widths are converted into the emission line to stellar bolometric luminosity ratio (R'). The 54 PMS stars have N_R < 10^{-1.0} and show R' \sim 10^{-4.2} as large as the maximum R' of the zero-age main-sequence (ZAMS) stars. However, because all R' was saturated against N_R, it was not possible to estimate the appropriate tau_c model for the PMS stars. We noticed that Mg I emission lines at 8808 A is an optically thin chromospheric line, appropriate for determination of the adequate tau_c for PMS stars. Using the archive data of the Anglo-Australian Telescope (AAT)/the University College London Echelle Spectrograph (UCLES), we investigated the Mg I line of 52 ZAMS stars. After subtracting photospheric absorption component, the Mg I line is detected as an emission line in 45 ZAMS stars, whose R' is between 10^{-5.9} and 10^{-4.1}. The Mg I line is not saturated yet in "the saturated regime for the Ca II emission lines", i.e. 10^{-1.6} < N_R < 10^{-0.8}. Therefore, the adequate tau_c for PMS stars can be determined by measuring of their R' values.

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arXiv2020

Measurements of the Ca II infrared triplet emission lines of pre-main-sequence stars

We investigated the chromospheric activity of 60 pre-main-sequence (PMS) stars in four molecular clouds and five moving groups. It is considered that strong chromospheric activity is driven by the dynamo processes generated by the stellar rotation. In contrast, several researchers have pointed out that the chromospheres of PMS stars are activated by mass accretion from their protoplanetary disks. In this study, the Ca II infrared triplet (IRT) emission lines were investigated utilizing medium- and high-resolution spectroscopy. The observations were conducted with Nayuta/MALLS and Subaru/HDS. Additionally, archive data obtained by Keck/HIRES, VLT/UVES, and VLT/X-Shooter was used. The small ratios of the equivalent widths indicate that Ca II IRT emission lines arise primarily in dense chromospheric regions. Seven PMS stars show broad emission lines. Among them, four PMS stars have more than one order of magnitude brighter emission line fluxes compared to the low-mass stars in young open clusters. The four PMS stars have a high mass accretion rate, which indicates that the broad and strong emission results from a large mass accretion. However, most PMS stars exhibit narrow emission lines. No significant correlation was found between the accretion rate and flux of the emission line. The ratios of the surface flux of the Ca II IRT lines to the stellar bolometric luminosity, R'_IRT, of the PMS stars with narrow emission lines are as large as the largest R'_IRT of the low-mass stars in the young open clusters. This result indicates that most PMS stars, even in the classical T Tauri star stage, have chromospheric activity similar to zero-age main-sequence stars.

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