More Like this

1. Nearly 30,000 Late-type Main-sequence Stars with Stellar Age from LAMOST DR5

   https://doi.org/10.3847/1538-4357/abce5b  

   Zhang, Jiajun; Zhao, Jingkun; Oswalt, Terry D; Liang, Xilong; Ye, Xianhao; Zhao, Gang (February 2021, The Astrophysical Journal)

   Abstract We construct a sample of nearly 30,000 main-sequence stars with 4500 K < T_eff < 5000 K and stellar ages estimated by the chromospheric activity−age relation. This sample is used to determine the age distribution in theR–Zplane of the Galaxy, whereRis the projected Galactocentric distance in the disk midplane andZis the height above the disk midplane. As ∣Z∣ increases, the percentage of old stars becomes larger. It is known that scale-height of Galactic disk increases asRincreases, which is called a flare. A mild flare fromR ∼ 8.0 to 9.0 kpc in stellar age distribution is found. We also find that the velocity dispersion increases with age as confirmed by previous studies. Finally we present spiral-shaped structures inZ–υ_Zphase space in three stellar age bins. The spiral is clearly seen in the age bin of [0, 1] Gyr, which suggests that a vertical perturbation to the disk probably took place within the last ∼1.0 Gyr. 

   more » « less
2. The Factory and the Beehive. V. Chromospheric and Coronal Activity and Its Dependence on Rotation in Praesepe and the Hyades

   https://doi.org/10.3847/1538-4357/ad117e  

   Núñez, Alejandro; Agüeros, Marcel_A; Curtis, Jason_L; Covey, Kevin_R; Douglas, Stephanie_T; Chu, Sabine_R; DeLaurentiis, Stanislav; Wang, Minzhi_Luna; Drake, Jeremy_J (February 2024, The Astrophysical Journal)

   Abstract Low-mass (≲1.2M_⊙) main-sequence stars lose angular momentum over time, leading to a decrease in their magnetic activity. The details of this rotation–activity relation remain poorly understood, however. Using observations of members of the ≈700 Myr old Praesepe and Hyades open clusters, we aim to characterize the rotation–activity relation for different tracers of activity at this age. To complement published data, we obtained new optical spectra for 250 Praesepe stars, new X-ray detections for 10, and new rotation periods for 28. These numbers for Hyads are 131, 23, and 137, respectively. The latter increases the number of Hyads with periods by 50%. We used these data to measure the fractional Hαand X-ray luminosities,L_Hα/L_bolandL_X/L_bol, and to calculate Rossby numbersR_o. We found that at ≈700 Myr almost all M dwarfs exhibit Hαemission, with binaries having the same overall color–Hαequivalent width distribution as single stars. In theR_o–L_Hα/L_bolplane, unsaturated single stars follow a power law with indexβ= −5.9 ± 0.8 forR_o> 0.3. In theR_o–L_X/L_bolplane, we see evidence for supersaturation for single stars withR_o≲ 0.01, following a power law with index ${\beta }_{\sup }={0.5}_{-0.1}^{+0.2}$, supporting the hypothesis that the coronae of these stars are being centrifugally stripped. We found that the criticalR_ovalue at which activity saturates is smaller forL_X/L_bolthan forL_Hα/L_bol. Finally, we observed an almost 1:1 relation betweenL_Hα/L_bolandL_X/L_bol, suggesting that both the corona and the chromosphere experience similar magnetic heating. 

   more » « less
3. Activity and Rotation of Nearby Field M Dwarfs in the TESS Southern Continuous Viewing Zone

   https://doi.org/10.3847/1538-3881/ac6110  

   Anthony, Francys; Núñez, Alejandro; Agüeros, Marcel A; Curtis, Jason L; do_Nascimento_Jr, J-D; Machado, João M; Mann, Andrew W; Newton, Elisabeth R; Rampalli, Rayna; Thao, Pa Chia; et al (May 2022, The Astronomical Journal)

   Abstract The evolution of magnetism in late-type dwarfs remains murky, as we can only weakly predict levels of activity for M dwarfs of a given mass and age. We report results from our spectroscopic survey of M dwarfs in the Southern Continuous Viewing Zone (CVZ) of the Transiting Exoplanet Survey Satellite (TESS). As the TESS CVZs overlap with those of the James Webb Space Telescope, our targets constitute a legacy sample for studies of nearby M dwarfs. For 122 stars, we obtained at least one R≈ 2000 optical spectrum with which we measure chromospheric Hαemission, a proxy for magnetic field strength. The fraction of active stars is consistent with what is expected for field M dwarfs; as in previous studies, we find that late-type M dwarfs remain active for longer than their early-type counterparts. While the TESS light curves for ≈20% of our targets show modulations consistent with rotation, TESS systematics are not well enough understood for confident measurements of rotation periods (P_rot) longer than half the length of an observing sector. We report periods for 12 stars for which we measure P_rot ≲ 15 days or find confirmation for the TESS-derived P_rot in the literature. Our sample of 21 P_rot, which includes periods from the literature, is consistent with our targets being spun-down field stars. Finally, we examine the Hα-to-bolometric luminosity distribution for our sample. Two stars are rotating fast enough to be magnetically saturated, but are not, hinting at the possibility that fast rotators may appear inactive in Hα. 

   more » « less
4. An X-ray activity cycle on the young solar-like star ɛ Eridani

   https://doi.org/10.1051/0004-6361/201936479  

   Coffaro, M.; Stelzer, B.; Orlando, S.; Hall, J.; Metcalfe, T. S.; Wolter, U.; Mittag, M.; Sanz-Forcada, J.; Schneider, P. C.; Ducci, L. (April 2020, Astronomy & Astrophysics)

   null (Ed.)

   Chromospheric Ca II activity cycles are frequently found in late-type stars, but no systematic programs have been created to search for their coronal X-ray counterparts. The typical time scale of Ca II activity cycles ranges from years to decades. Therefore, long-lasting missions are needed to detect the coronal counterparts. The XMM-Newton satellite has so far detected X-ray cycles in five stars. A particularly intriguing question is at what age (and at what activity level) X-ray cycles set in. To this end, in 2015 we started the X-ray monitoring of the young solar-like star ɛ Eridani, previously observed on two occasions: in 2003 and in early 2015, both by XMM-Newton . With an age of 440 Myr, it is one of the youngest solar-like stars with a known chromospheric Ca II cycle. We collected the most recent Mount Wilson S-index data available for ɛ Eridani, starting from 2002, including previously unpublished data. We found that the Ca II cycle lasts 2.92 ± 0.02 yr, in agreement with past results. From the long-term XMM-Newton lightcurve, we find clear and systematic X-ray variability of our target, consistent with the chromospheric Ca II cycle. The average X-ray luminosity is 2 × 10 28 erg s −1 , with an amplitude that is only a factor of 2 throughout the cycle. We apply a new method to describe the evolution of the coronal emission measure distribution of ɛ Eridani in terms of solar magnetic structures: active regions, cores of active regions, and flares covering the stellar surface at varying filling fractions. Combinations of these three types of magnetic structures can only describe the observed X-ray emission measure of ɛ Eridani if the solar flare emission measure distribution is restricted to events in the decay phase. The interpretation is that flares in the corona of ɛ Eridani last longer than their solar counterparts. We ascribe this to the lower metallicity of ɛ Eridani. Our analysis also revealed that the X-ray cycle of ɛ Eridani is strongly dominated by cores of active regions. The coverage fraction of cores throughout the cycle changes by the same factor as the X-ray luminosity. The maxima of the cycle are characterized by a high percentage of covering fraction of the flares, consistent with the fact that flaring events are seen in the corresponding short-term X-ray lightcurves predominately at the cycle maxima. The high X-ray emission throughout the cycle of ɛ Eridani is thus explained by the high percentage of magnetic structures on its surface. 

   more » « less
5. A New Magnetic Parameter of Active Regions Distinguishing Large Eruptive and Confined Solar Flares

   https://doi.org/10.3847/2041-8213/ac5251  

   Li, Ting; Sun, Xudong; Hou, Yijun; Chen, Anqin; Yang, Shuhong; Zhang, Jun (February 2022, The Astrophysical Journal Letters)

   Abstract With the aim of investigating how the magnetic field in solar active regions (ARs) controls flare activity, i.e., whether a confined or eruptive flare occurs, we analyze 106 flares of Geostationary Operational Environmental Satellite class ≥M1.0 during 2010–2019. We calculate mean characteristic twist parameters α FPIL within the “flaring polarity inversion line” region and α HFED within the area of high photospheric magnetic free energy density, which both provide measures of the nonpotentiality of the AR core region. Magnetic twist is thought to be related to the driving force of electric current-driven instabilities, such as the helical kink instability. We also calculate total unsigned magnetic flux (Φ AR ) of ARs producing the flare, which describes the strength of the background field confinement. By considering both the constraining effect of background magnetic fields and the magnetic nonpotentiality of ARs, we propose a new parameter α /Φ AR to measure the probability for a large flare to be associated with a coronal mass ejection (CME). We find that in about 90% of eruptive flares, α FPIL /Φ AR and α HFED /Φ AR are beyond critical values (2.2 × 10 −24 and 3.2 × 10 −24 Mm −1 Mx −1 ), whereas they are less than critical values in ∼80% of confined flares. This indicates that the new parameter α /Φ AR is well able to distinguish eruptive flares from confined flares. Our investigation suggests that the relative measure of magnetic nonpotentiality within the AR core over the restriction of the background field largely controls the capability of ARs to produce eruptive flares. 

   more » « less