Edward F. Guinan

α ¹ Her is a ∼3rd-mag semi-regular/irregular, luminous M5 Ib-II red giant and one of the nearest asymptotic giant branch (AGB) stars. It is the brightest member of a triple-star system, fortuitously permitting well-constrained age and fundamental properties. We investigate the star’s pulsation and convection-driven light variations utilizing ∼80 yr of visual measures and over a decade of photoelectric/CCD V -band observations from the American Association of Variable Star Observers (AAVSO) database. Period analyses were conducted using Weighted Wavelet Z-transform (WWZ) and CLEANest programs from the Peranso-3 period-analysis software. Analysis of the visual data reveals low-amplitude (∼0.2–0.4 mag) cycles of ∼1360 day, and ∼9–10 yr, along with an apparent long-term brightness increase of ∼0.2 mag. Investigations of over ∼13 yr of V -band observations find complex periodicities of ∼123, ∼200, and ∼390–490 day, along with a ∼9–10 yr oscillation.

Two of the most promising, potentially habitable and nearby exoplanets orbit Teegarden’s Star, a M7V red dwarf, only 12.5 lt-yr away. We determine the ages and the photoionizing and photodissociating X-ray/FUV (XUV) irradiances of these rocky, habitable zone (HZ) Earth-size exoplanets: Teegarden-b: M sin i ( M ⊕ ) = 1.16 ; T _eq (K) = 277 and Teegarden-c: M sin i ( M ⊕ ) = 1.11 ; T eq ( K ) = 209 . Teegarden-d was recently reported well beyond the outer edge of the HZ. Age is estimated from the star’s rotation (gyrochronology), UVW space-motions, Activity–Age relations and metallicity. A weighted mean age ∼7.4 ± 1.8 Gyr is determined. The XUV irradiances are compared to Proxima-b and the TRAPPIST-1 system. These planetary XUV irradiances are important since they strongly affect the hosted planets’ ability to retain atmospheres and water inventories crucial for life.

We present the first Hubble Space Telescope - Space Telescope Imaging Spectrograph (HST-STIS) far-ultraviolet (FUV) and near-ultraviolet (NUV) high-resolution spectra of the red supergiant (RSG) Antares (M1.5 Iab), isolated from its long-period B-star binary companion, and compare them to spectra of Betelgeuse (M2 Iab) to assess the similarities and differences in chromospheric heating and atmospheric dynamics. While Betelgeuse has two well-known circumstellar CO outflows, previous radio and infrared observations of Antares did not detect the presence of circumstellar CO. The Antares FUV STIS spectra reveal CO electronic Fourth-Positive absorption bands formed exterior to the chromosphere. The derived column density is 4 orders of magnitude smaller than the CO MOLsphere located interior to 1.7 R _* . The numerous NUV Fe ii emission lines show classical wind-scattering profiles, with velocities close to the star of v _wind ≃ 19 km s ⁻¹ , consistent with that previously inferred from absorption spectra observed against the B star at a projected radius of ∼150 R _* . Surprisingly, the NUV semi-forbidden emission lines show evidence of wind scattering, indicative of the massive column densities present in the winds of low-surface-gravity RSGs. Differential comparison with Betelgeuse of chromospheric fluxes from emission lines excited by electron collisions, electron collisions combined with photospheric photoexcitation, and coincidental line pumping reveals similar levels of chromospheric heating. C ii ] indicates comparable chromospheric electron densities and column densities. With its well-constrained distance and stellar parameters, Antares provides a good source for understanding the processes driving RSG outflows. Chromospherically, Antares is effectively a twin of Betelgeuse.

CE Tau (119 Tauri) is a 4th-magnitude semi-regular variable M2 Iab-Ib red supergiant with physical properties similar to Betelgeuse. Using the precise Gaia DR3 parallax, the properties of CE Tau are updated. We investigate the star’s pulsation and convective properties utilizing (∼5400+) visual measures from 1954 to 2026 and (∼1100+) photoelectric/CCD V -band photometry from 1981. Data are primarily from the American Association of Variable Star Observers database. Period analyses were performed using the CLEANest power spectrum and Weighted Wavelet Z-transform programs from the Peranso-3 period-analysis software. Analysis of visual-photometry returns a period of ∼1300 days with transitory periodicities of ∼380 and ∼9200 days (∼25.3 yr). Over well covered regions of photoelectric and CCD V -band subsets, analysis yields a dominant mean period of ∼162 ± 5 days with light amplitudes ranging from ∼0.10 to 0.25 mag. Also present are transitory periodicities of ∼1300 ± 30 and ∼403 ± 20 days.

We report the resolution of a halo of X-ray line emission surrounding the zero-age main-sequence G8.5V star HD 61005 by Chandra/ACIS-S. Located only ∼36 pc distant, HD 61005 is young (100 ± 50 Myr) and X-ray bright (∼300 × solar), observed with a nearly edge-on geometry and surrounded by very local interstellar medium (VLISM) material denser than in the Sun’s environs. HD 61005 is known to harbor large amounts of circumstellar dust in a dense ecliptic plane full of millimeter-sized particles plus attached, extended “Wing like structures” full of micron-sized particles, which are evidence for strong VLISM–dust disk interaction. These properties aided our ability to resolve the system’s ∼220 au wide astrosphere, the first ever observed for a main-sequence G star. The observed X-ray emission morphology is roughly spherical, as expected for an astrospheric structure dominated by the host star. The Chandra spectrum of HD 61005 is a combination of a hard stellar coronal emission ( T ∼ 8 MK) at L _X ∼ 6 × 10 ²⁹ erg s ⁻¹ , plus an extended halo contribution at L _X ∼ 1 × 10 ²⁹ erg s ⁻¹ dominated by charge exchange (CX) lines, such as those of O VIII and Ne IX . The Chandra CX X-ray morphology does not track the planar dust morphology but does extend out roughly to ∼110 au where the base of the dust wings begins. We present a toy model of the astrosphere emission produced by stellar wind–VLISM CX interactions, similar to the state of the young Sun when it was only ∼10 ⁸ yr old and transiting through an ∼10 ³ times denser part of the interstellar medium (such as a giant molecular cloud).

Betelgeuse is the brightest red supergiant visible from Earth. Betelgeuse has nearly 200 yr of visual observations as well as systematic photoelectric and CCD data that indicate semi-regular light variations. In mid-February 2020, Betelgeuse surprisingly faded to its faintest and coolest ever recorded—dubbed the “Great Dimming” (V +1.61 mag). Previously, Betelgeuse had multiple periodicities with dominant periods of 185, 380–420, and 2000–2200 days. We find that even before this dimming, a new dominant period of 208 ±12 days superseded the 400 days periodicity. Additionally present is a 2000–2200 days Long Secondary Period (LSP) that remains consistent throughout the dimming. The LSP has recently been proposed to arise from a close-orbiting binary companion star. Contrary to expectation, however, the LSP light minimum appears to be occurring several months earlier than predicted. We present results of quantitative period analyses and draw tentative conclusions.

Antares A is a bright ∼0.6–1.3 mag irregular/semi-regular variable M1.5 Iab star. Antares with Betelgeuse (M2.5 Iab) are the two nearest red supergiant, core-collapse Type-II supernova progenitors. Unlike Betelgeuse, Antares A has a resolved companion (B4V: Antares B) and is near its birthplace in the Upper Scorpius OB Association. We analyzed ∼70 yr of photometry mostly from the American Variable Star Observers database and Hipparcos. Due to its southern declination (−26 . ° 4), accurate visual photometry from mid-northern latitudes is difficult where air masses often exceed z > 3.0. As a result, Antares has fewer, generally lower quality visual-magnitude measurements. We vetted the observations, applying less weight to the visual observations made at high airmasses. The photoelectric observations were given higher weights. Period analyses were conducted utilizing the CLEANest Power-Spectrum and weighted wavelet Z-transform Wavelet programs provided in the Peranso-3 period analysis software. The results are compared to Betelgeuse.

Recent studies indicate that Gliese 581d, a proposed habitable zone (HZ) super-Earth planet, does not exist, as the respective data denote that the planet is an artifact of stellar activity. Here we report evidence to the contrary considering that those studies were based on inaccurate spectroscopic measurements of the stellar rotation period regarding the planet’s inactive host star (dM3). Gliese 581d, if real, is of particular interest as it constitutes the first planet identified to be in a stellar HZ outside of the solar system based on studies in 2007. If confirmed as a true planet, at 20.5 lt-yr, it would also be one of the nearest potentially habitable super-Earths.

Age is a fundamental stellar property, yet for many stars, it is difficult to reliably determine. For M dwarfs, it has been notoriously so. Due to their lower masses, core hydrogen fusion proceeds at a much slower rate in M dwarfs than it does in more massive stars like the Sun. As a consequence, more customary age determination methods (e.g., isochrones and asteroseismology) are unreliable for M dwarfs. As these methods are unavailable, many have searched for reliable alternatives. The M dwarfs comprise the overwhelming majority of the nearby stellar inventory, which makes the determination of their fundamental parameters even more important. Further, an ever-increasing number of exoplanets are being found to orbit M dwarfs, and recent studies have suggested that they may have a relatively higher number of low-mass planets than other spectral types. Determining the ages of M dwarfs then allows us to better study any hosted exoplanets as well. Fortunately, M dwarfs possess magnetic activity and stellar winds like other cool dwarf stars. This causes them to undergo the spin-down effect (rotating with longer periods) as they age. For this reason, stellar rotation rate has been considered a potentially powerful age determination parameter for over 50 yr. Calibrating reliable age–rotation relationships for M dwarfs has been a lengthy process, but here we present the age–rotation relationships for ∼M0–6.5 dwarfs, determined as part of the Living with a Red Dwarf program. These relationships should prove invaluable for a wide range of stellar astrophysics and exoplanetary science applications.

GJ 667Cc is a nearby (23.6 lt-yr) super-Earth that orbits within the habitable zone (HZ) of its ∼M2V host star and has Earth-like properties: (M- sin ( i ) ∼ 4.1 M ⊕ ; R ∼ 1.7 R _⊕; T _eq (A = 0.3) ∼ 277 K). The age of the star/planet is poorly constrained at 2 ≳ Gyr. Age is crucial in evaluating the potential for complex life. We determine an age of ∼6.10 ± 2.2 Gyr from our recent Age-Rotation relations. This is compatible with its low metallicity ([Fe/H] ∼ −0.59 ± 0.1 dex), weak coronal/chromospheric X-ray/FUV emissions, and space motions. We also determine the photodissociating/photoionizing X-ray-UV irradiances from Chandra and ROSAT X-ray and HST/FUV observations. Like most HZ-planets hosted by M-dwarfs, these irradiances are much higher than Earth's and are detrimental to the planet's atmosphere retention, water inventories, and habitability. A strong geomagnetic field could mitigate atmospheric erosion, permitting life to develop.