Evolution Of Stellar Activity Research Articles

The evolution of the solar–stellar activity

We present a brief review of observational results contributing to modern ideas on the evolution of stellar activity. Basic laws, derived for both rotation–age and activity–rotation relationships, allow us to trace how the activity of low-mass stars changes with age during their stay on the main sequence. We focus on the evaluation of the activity properties of stars that could be analogs of the young Sun. Our study includes joint consideration of different tracers of activity, rotation and magnetic fields of Sun-like stars of various ages. We identify rotation periods, when the saturated regime of activity changes to the unsaturated mode, when the solar-type activity is formed: for G- and K-type stars, they are 1.1 and 3.3 days, respectively. This corresponds to an age interval of about 0.2–0.6 Gyr, when regular sunspot cycle began to be established on the early Sun. We discuss properties of the coronal and chromospheric activity in young Suns. Our evaluation of the EUV-fluxes in the spectral range of 1350−1750 Å shows that the far-UV radiation of the early Sun was a factor of 7 times more intense than that of the present-day Sun, and twice higher when the regular sunspot cycle was established. For the young Sun, we can estimate the possible mass loss rate associated with quasi-steady outflow as 10−12 M⊙/yr. The results of observations of the largest flares on solar-type stars are also discussed, leading to conclusion that the most powerful phenomena occur on the fast-rotating stars in the saturated activity regime. Our estimate of the stellar magnetic fields makes it possible to evaluate the maximal possible flare energy. This could help us better understand the origin of extreme events on the Sun in the past.

Observational constraints for solar-type Stellar winds

AbstractIonized winds from late-type main-sequence stars are important for stellar spin-down and therefore the evolution of stellar activity; winds blow an “astrosphere” into the interstellar medium that absorbs a large part of galactic cosmic rays; and the winds play a key role in shaping planetary environments, in particular their upper atmospheres. These issues have been well studied for the solar wind but little is known about winds escaping from other solar-type stars. Several methods have been devised to either detect winds directly or to infer the presence of such winds from features that are shaped by the winds. This paper summarizes these methods and discusses exemplary findings. There is need for more studies using multiple methods for the same stars.

The X-ray luminosity of solar-mass stars in the intermediate age open cluster NGC 752

Aims. While observational evidence shows that most of the decline in a star’s X-ray activity occurs between the age of the Hyades (∼8 × 10 8 yr) and that of the Sun, very little is known about the evolution of stellar activity between these ages. To gain information on the typical level of coronal activity at a star’s intermediate age, we studied the X-ray emission from stars in the 1.9 Gyr old open cluster NGC 752. Methods. We analysed a ∼140 ks Chandra observation of NGC 752 and a ∼50 ks XMM-Newton observation of the same cluster. We detected 262 X-ray sources in the Chandra data and 145 sources in the XMM-Newton observation. Around 90% of the catalogued cluster members within Chandra’s field-of-view are detected in the X-ray. The X-ray luminosity of all observed cluster members (28 stars) and of 11 cluster member candidates was derived. Results. Our data indicate that, at an age of 1.9 Gyr, the typical X-ray luminosity of the cluster members with M = 0.8−1.2 M� is LX = 1.3 × 10 28 erg s −1 , so approximately a factor of 6 less intense than that observed in the younger Hyades. Given that LX is proportional to the square of a star’s rotational rate, the median LX of NGC 752 is consistent, for t > 1 Gyr, to a decaying rate in rotational velocities vrot ∝ t −α with α ∼ 0.75, steeper than the Skumanich relation (α � 0.5) and significantly steeper than observed between the Pleiades and the Hyades (where α< 0.3), suggesting that a change in the rotational regimes of the stellar interiors is taking place at t ∼ 1G yr.

Evolution of the symbiotic star AS 338 after its strong outburst in 1983

The photometric UBV observations of AS 338 that we began after its outburst in 1983 are presented. They were accompanied by yearly spectroscopic observations and by occasional estimations of the star’s infrared JHKL magnitudes. In June 1993, the star’s optical spectrum was extended to the ultraviolet via IUE observations of AS 338. Collectively, the above observations make it possible to trace the evolution of stellar activity over a period of 15 years in various spectral ranges. In particular, a short-time return of the hot component of AS 338 to the state when He II lines reappeared in the star’s spectrum was noted in 1993. At this time, a blend of the C IV λλ5802 and 5812 lines, which is typical of Wolf-Rayet spectra, was detected in it. In June 1993, the temperature of the hot component was Th ≈ 8.8 × 104 K, and the ratio of its bolometric flux to that of the red giant was Fh, bol/Fg, bol ≈ 1.0. In August, its temperature increased to ∼1.0×105 K, while the bolometric flux dropped by a factor of ∼1.5(Fh, bol/Fg, bol ≈ 0.7). In the B-V, U diagram, the points referring to this so-called quiescent state form a separate group shifted in B-V from all the remaining ones located in a horizontal strip with \(\Delta U \approx 3\mathop .\limits^m 5\) and \(\Delta (B - V) \approx 0\mathop .\limits^m 4\). This allows us to diagnose the state of the hot component without spectroscopic observations of the star. In October 1993, the hot component flared up again. The main brightness rise took no more than 19 days. The outburst occurred shortly before eclipse egress of the hot component, whose duration was ∼0.01Porb. In December 1993, Fh, bol/Fg, bol≤1.5 at maximum light. During the recurrent, even stronger outburst in April 1995, Fh, bol/Fg, bol≤3.4. The Hαline during outbursts has a P Cyg profile and broad wings stretching to velocities of ±1500 km s−1. The color temperature of the active hot component at short optical wavelengths and in the ultraviolet lies in the range of effective temperatures for hot supergiants. Nevertheless, it always produces an H II region in the circumstellar envelope that is larger in size than this binary system.

Evolution of Stellar Activity in Early Post-Main-Sequence Phases

We present preliminary observational evidences on the variation of the activity level in late type stars, during the evolutionary phases on the main sequence and beyond. We have selected a sample of 51 stars (Fig. 1), lying mostly along evolutionary tracks between 1.3 and 1.7 solar masses, which have been observed in soft X-rays with the Einstein Observatory, and in UV with IUE (Maggio et al. 1990; Haisch et al.1990). Two ROSAT targets, and four new IUE observations are also included.We find that the radiative emission from the outer atmospheres of stars with M &gt; 1.6M⊙ seems to behave differently than for stars with lower mass.On the main sequence, the X-ray luminosity of most stars with B-V &lt; 0.42 (spectral type F3) is relatively low, at Lx ~ 3 × 1028erg s-1 (Fig. 2). In the early evolutionary phases beyond the main sequence, the X-ray luminosity of the higher mass stars tend to increase sistematically up to ~ 1030erg s-1, while the lower mass stars show an initial moderate increase followed by a drop, at B-V ~ 0.6, below our sensitivity threshold.

The distant future of solar activity: A case study of Beta Hydri. I - Stellar evolution, lithium abundance, and photospheric structure

The post-main-sequence evolution of stellar activity and of nonthermal processes in solar-type atmospheres is studied through a detailed comparison of the current Sun (G2 V) with the very old solar-type star β Hyi (G2 IV). Magnetically related activity decreases with age, and in very old stars one might possibly find the absolute minimum of stellar activity. In the solar galactic neighborhood, the closest single solar-type star, with an old age well-determined from evolutionary tracks, is β Hyi (HR 98, HD2151; G2 IV)