Commentary – Cool Star Videos (spotted stars)

Starspots, like Sunspots, basically trace the strength of the magnetic flux over the surfaces of stars. Doppler imaging of these features on stars gives us information about spot shapes, distributions, and variability along with the differential rotation of the star itself. In this way we can make comparisons with the Sun’s activity to assist us in understanding the behavior of the most important star, the Sun.

Here we show examples of spotted stars at stages in their life cycle before they are on the main sequence (i.e. before core hydrogen burning sets in), just arrived on the main sequence and after the main sequence when they are in a giant phase.

T Tauri stars are a class of very young stars still in the process of gravitational contraction before arriving on the main-sequence. They are generally 100,000 to a 10 million years in age since contraction began, they are generally 0.5 to 3 times the Sun’s mass and are losing mass through a stellar wind from their surface. They usually appear in the dark clouds from which they formed but some appear as a “naked T Tauri”, that lacks a surrounding dense cloud of gas and dust. Some of these naked T Tauri stars have magnetic activity indicated by starspots on their surfaces. One such T Tauri is V410 Tau that we have discussed briefly below and show as a video image.

Very young main-sequence stars that have just settled into the core hydrogen burning phase of their life and that have masses of approximately a solar mass or less often show large starspots indicating considerable magnetic activity. Several examples of these are discussed below briefly and in particular a binary pair (sigma-2 CrB) where the two stars were imaged simultaneously from their combined spectrum variations.

RS CVn stars are late-type evolved stars of about spectral type K (slightly cooler surface temperature than the Sun). These stars are found in binary systems, often where the binary companion is so faint as to be virtually invisible. The fact they are cool stars means they have a deep convection zone and the tidal coupling with their companion forces the star to rotate at high velocity leading to very strong magnetic activity characterized by large starspots. These stars can have as much as 20% or more of their surface covered with spots compared with a few percent on the Sun.

The FK Comae stars, like the RS CVn stars, are evolved stars with strong magnetic activity but unlike the RS CVn stars they appear to be single stars. They may be merged binaries where the high spin rate derives from the original binary nature of the stars.

 

T Tauri type star

V410 Tau

V410 Tau is a weak lined T Tauri star we imaged using spectra from the Canada France Hawaii Telescope. The star was observed on two occasions separated by about a year (see Astronomy and Astrophysics 316, 164, 1996 compared with Astronomy and Astrophysics 285, L17, 1994) and the images revealed that the pattern of spots was persistent enough to allow us to identify common features that lasted for the full year. By comparing the motions of features near the equator and near the pole we could establish both the sign and magnitude of the differential rotation. The differential rotation of this star was seen to be very small compared to the Sun and in the same sense as solar differential rotation (i.e. the equatorial regions rotate faster than the polar regions).

At the bottom of the two videos of V410 tau shown here you will see the variation in brightness displayed by V410 Tau as it rotated in 1993 compared with 2008. Note that in December of 1993, the brightness variation of V410 Tau was almost a factor of two from minimum to maximum as a result of the asymmetric  positioning of the stellar spots but as can be seen from the image of 2008 when the star was once more observed, the light variation was almost non-existent and the spots more uniformly distributed. This may be half of the full cycle of what is often called the flip-flop behavior of stellar spot distribution where spots alternate their distribution on the stellar surface between two longitudes separated by 180 degrees (Astrophysical Journal 728, 69, 2011).

 

Young main sequence solar like stars

Sigma-2 Coronae Borealis

This is a Doppler Image generated simultaneously from high resolution spectra taken of this double line binary at the Canada France Hawaii Telescope. Both of these stars are very close to the size and temperature of the Sun being dwarf stars at F9 and G0 spectral type. A striking feature is that the cool spots appear at polar latitudes and perhaps especially that each star has a warm, more equatorial belt on the trailing hemisphere. (Astronomy and Astrophysics 399, 315, 2003)

LQ Hydrae and EK Draconis

LQ Hya is a very young, single K2-dwarf star that illustrates well the considerable activity of a star somewhat less massive than the Sun and recently arrived on the main-sequence. The cool spots are, perhaps like the Sun, concentrated in a band (or two close bands) in the equatorial region but in addition there seems to be a weak polar feature as is common with the more evolved stars. The cool spot “patches” on LQ Hya may represent clusters of smaller spots that are unresolved with the reduced resolution given the somewhat lower Vsini of LQ Hya compared with other examples of stars shown here. (Astronomy and Astrophysics 268, 671, 1993, Astronomy and Astrophysics 336, 972, 1998 and Astronomy and Astrophysics 373, 199, 2001))

EK Drac is another young main sequence star. The principle feature on this star is a large polar spot. Given the relatively quite small Vsini of this star and the Doppler Imaging limits the strength and existence of this spot can be viewed with some reservation but it is a very likely feature. When the images of LQ Hya and EK Drac are compared with theoretical predictions for the locations of spots on such young stars of roughly solar mass, it appears surprising that the observations for LQ Hya and EK Drac are not reversed given that LQ Hya is apparently significantly less massive than EK Drac and should have a deeper convective envelope to allow strong polar spot development. (Astronomy and Astrophysics 330, 685, 1998)

 

The FK Comae type stars

HD 199178 is an FK Comae type star. There are three images of HD 199178 shown here that were taken in 1990, and two in 1989 (April and May-June). These are from the paper Astronomy and Astrophysics 347, 212, 1999 where there are actually five separate images reported and discussed (1988 to 1997). The prototype for these stars is, of course, FK Comae itself. The general description of the type is that the stars are a group of rapidly rotating single giants. The stars are located in the Hertzsprung gap of the H-R diagram and have in common strong magnetic activity from the radio to the X-ray wavelengths.

All images of HD 199178 are characterized by a large polar spot and several low latitude spots with spot temperatures for the low latitude spots about 710 K below photosphere and for the polar spot about 1700 K below photosphere. Lifetime of the low latitude spots was found to be as low as about one month but the polar spot lifetime exceeded our observing period of 9 years and longer since other observers had mapped it as early as 1985.

 

RS Canum Venaticorum  stars

HU Virgo (HD 106225)

HU Virgo is an RS Canum Venaticorum type star. This Doppler Image shows a star with a large cool polar spot and two broad more equatorially located plage regions that are 180 degrees of longitude apart. The region between these two plage regions is occupied by appendages of the polar spot. Multiple data sources (Ca II, H&K and H alpha profile variations along with photometric broad band colours and line depth ratios) provided the basis for an analysis of the chromospheric structure and flows in depth over the surface of the star. This information was related to the photosphere as shown from this Doppler Image. The details are in the paper Astronomy and Astrophysics 281, 395, 1994.

EI Eri

The first of the cool stars to be observed with our Doppler Imaging technique (Astronomy and Astrophysics 247, 130, 1991). This is an evolved G5IV RS C Vn star. The startling thing about this star, at the time, was the appearance of a huge polar spot. This challenged the sense that polar spots would not be seen, a view strongly supported by the lack of polar spots ever appearing on the Sun. Any attempt to model the variations in the spectral line profiles without a strong polar spot feature failed completely and the discovery of many other stars with polar spots subsequently confirmed that they are a rather common feature on many of the larger evolved cool stars and even among the main sequence young stars (see EK Draconis video)

XX Tri is another of the RS CVn stars. A K0 giant with a rotation period of 24 days that has been observed by my colleagues at the Liebnitz Institute in Potsdam over six years using the robotic telescopes they operate in the Canary Islands. In their paper (Astronomy and Astrophysics 578, A101, 2015) they describe large polar spots and transient smaller equatorial spots with a systematically changing distribution along with spot fragmentation and merging. They see a weak solar like differential rotation and predict an activity cycle of about 26 years. There is an active longitude in phase toward the unseen companion. They have generated a beautiful time series video of XX Tri over the six years.