Starspot Temperatures Research Articles

Star-spots and magnetism: testing the activity paradigm in the Pleiades and M67

ABSTRACT We measure star-spot filling fractions for 240 stars in the Pleiades and M67 open star clusters using APOGEE high-resolution H-band spectra. For this work, we developed a modified spectroscopic pipeline which solves for star-spot filling fraction and star-spot temperature contrast. We exclude binary stars, finding that the large majority of binaries in these clusters (80 per cent) can be identified from Gaia DR3 and APOGEE criteria – important for field star applications. Our data agree well with independent activity proxies, indicating that this technique recovers real star-spot signals. In the Pleiades, filling fractions saturate at a mean level of 0.248 ± 0.005 for active stars with a decline at slower rotation; we present fitting functions as a function of Rossby number. In M67, we recover low mean filling fractions of 0.030 ± 0.008 and 0.003 ± 0.002 for main sequence GK stars and evolved red giants, respectively, confirming that the technique does not produce spurious spot signals in inactive stars. Star-spots also modify the derived spectroscopic effective temperatures and convective overturn time-scales. Effective temperatures for active stars are offset from inactive ones by −109 ± 11 K, in agreement with the Pecaut & Mamajek empirical scale. Star-spot filling fractions at the level measured in active stars changes their inferred overturn time-scale, which biases the derived threshold for saturation. Finally, we identify a population of stars statistically discrepant from mean activity–Rossby relations and present evidence that these are genuine departures from a Rossby scaling. Our technique is applicable to the full APOGEE catalogue, with broad applications to stellar, galactic, and exoplanetary astrophysics.

Hiding in plain sight: observing planet-starspot crossings with the James Webb Space Telescope

ABSTRACT Transiting exoplanets orbiting active stars frequently occult starspots and faculae on the visible stellar disc. Such occultations are often rejected from spectrophotometric transits, as it is assumed they do not contain relevant information for the study of exoplanet atmospheres. However, they can provide useful constraints to retrieve the temperature of active features and their effect on transmission spectra. We analyse the capabilities of the James Webb Space Telescope in the determination of the spectra of occulted starspots, despite its lack of optical wavelength instruments on board. Focusing on K and M spectral types, we simulate starspots with different temperatures and in different locations of the stellar disc, and find that starspot temperatures can be determined to within a few hundred kelvins using NIRSpec/Prism and the proposed NIRCam/F150W2+F322W2’s broad wavelength capabilities. Our results are particularly promising in the case of K and M dwarfs of mag$_K \lesssim 12.5$ with large temperature contrasts.

On the relationship between the size and surface coverage of starspots on magnetically active low-mass stars

We present a model that predicts the light curve amplitude distribution for an ensemble of low-mass magnetically active stars, under the assumptions that stellar spin axes are randomly orientated and that cool starspots have a characteristic scale length and are randomly distributed across the stellar surfaces. The model is compared with observational data for highly magnetically active M-dwarfs in the young cluster NGC 2516. We find that the best fitting starspot scale length is not constrained by these data alone, but requires assumptions about the overall starspot filling factor and starspot temperature. Assuming a spot coverage fraction of 0.4+/-0.1 and a starspot to unspotted photosphere temperature ratio of 0.7+/-0.05, as suggested by the inflated radii of these stars compared to evolutionary model predictions and by TiO band measurements on other active cool stars of earlier spectral type, the best-fitting starspot angular scale length is 3.5(+2)(-1) degrees, or a linear scale length of \sim 25000 km. This linear scale length is similar to large sunspot groups, but 2--5 times smaller than the starspots recently deduced on an active G-dwarf using eclipse mapping by a transiting exoplanet. However, the best-fitting spot scale length in the NGC 2516 M-dwarfs increases with the assumed spot temperature ratio and with the inverse square root of the assumed spot filling factor. Hence the light curve amplitude distribution might equally well be described by these larger spot scale lengths if the spot filling factors are <0.1 or the spot temperature ratio is >0.9.

Photometric evidence for two-temperature photospheric inhomogeneities on magnetically active K dwarf stars

We derive the properties of spotted regions in late-type active stars from V - and B -band photometric data. Specifically, we compare the amplitudes of spot-induced light and color variations with model amplitudes, which depend on the area, temperature and distribution of spotted regions over the stellar surface. Our analysis is applied to three well-known chromospherically active K dwarf stars: DX Leonis, AB Doradus and LQ Hydrae, and indicates that all three stars show significant variations of the average spot temperature from epoch to epoch. The observed temporal variations of the starspot temperatures are best explained as arising from the contemporary presence on the stellar photosphere of two-component (two-temperature) active regions. These consist of either dark spots and hot faculae and/or dark umbrae and penumbrae and the variations over time arise from change of their areal ratio.

Measuring starspot temperature from line-depth ratios

We present and apply to VY Ari, IM Peg and HK Lac a new method to determine spot temperatures (T sp ) and areas (A rel ) from the analysis of simultaneous light curves and temperature modulations deduced from line-depth ratios. A spot model, developed by us, has been applied to light and temperature curves. Grids of solutions with comparable χ -square have been found for a wide range of spot temperatures. The behavior of the solution grids for temperature and light curves in the T sp –A rel plane is very different and a rather small and unique intersection area can be found. In our spot-model we used spectral energy distributions (SEDs) based on the Planck law and on model atmospheres to evaluate the flux ratio between spots and unspotted photosphere and we found higher spot temperatures with SEDs based on model atmospheres than on the Planck law.

Measuring starspot temperature from line depth ratios

Gray and collaborators have recently demonstrated that line-depth ratios are a powerful tool for temperature discrim- ination, able to resolve dierences10 K. The method has been applied to detect temperature variations in the 5-15 K range due to activity cycles (e.g. Gray et al. 1996a, 1996b) or to rotation modulation produced by large surface features, called star-patches, like that detected in Boo A by Toner & Gray (1988). Cool starspots of a few tenths of the stellar surface produce bumps in a line profile, which migrate through the line profile allowing Doppler-imaging in fast rotating stars. In the hypothesis that in slowly-rotating stars the passage of dark spots pro- duces modulation of the center line depth of dierent amount in lines of dierent sensitivity to temperature, we have made test observations on three active binaries of the RS CVn type. Based on observations made at the Catania Astrophysical Observatory at a resolution R= 14 000, we show that line-depth ratios can be eectively used to determine spot temperatures of active binary systems. Using, on average, ten line pairs, selected in the 6100-6300 A wavelength range, with the help of observations of 30 main sequence and giant stars, we have derived a calibration relation of line-depth ratios (LDR) in an absolute temperature scale, taking into account the gravity eect in the calibration relation. Single LDRs converted to temperature through the calibration relations have led to clear rotational modulation of the average surface temperature with amplitudes of 177 K, 119 K, and 127 K for VY Ari, IM Peg and HK Lac, with average estimated errors of about 10 K. We show that the observed temperature variation amplitude allows us to define a minimum fractional spotted area coverage as a function of spot-photosphere temperature ratio. Adopting the maximum value of average temperature, determined from the LDRs, as that of the unspotted photosphere, we computed the average spot temperature corresponding to the minimum spot coverage. Although not univocally constrained, the temperature dierence (T = Tph Tsp) obtained for the three systems,T= 890 K for VY Ari,T= 750 K for IM Peg, andT= 810 K for HK Lac, are in good agreement with values derived with other methods.

Variations in Spottedness of Photosphere and Chromosphere of the Flare Star EV Lac

AbstractQuantitative analysis of the variations of EV Lac as being caused by spottedness leads to the conclusion that the total areas of starspots vary strongly – up to two or threefold – from one season to another but the starspot temperatures remain constant.

A method for unambiguous determination of starspot temperatures and areas - Application to II Pegasi, BY Draconis, and HD 209813

A method is presented for the unique derivation of both starspot temperatures and effective areas from the range of standardized V and R light curves obtained through the rotation periods, and the method is applied to spots observed on II Pegasi, BY Draconis, and HD 209813. The method involves the determination of the temperature difference between the spot and the surrounding photosphere from V and V-R photometry and the Barnes-Evans visual surface brightness relation, and area determination by the derivation of a parameterized effective area representing the fractional area of disk covered by spots under the assumption that the spot or spot group is adequately described by a single circular spot. By this method, an effective temperature of 3400 K and an area representing 37% of the facing hemisphere are derived for the large, cool spot observed on II Pegasi during 1977, an effective temperature of 3500 K and area of 34% are derived for BY Dra and an effective temperature of less than 3840 K is obtained for HD 209813. The derived starspot temperatures, which are believed to correspond to the starspot penumbrae, imply umbral temperatures of 2600-2900 K, in excellent agreement with the 3 kilogauss umbral spot models of Mullan (1974). The geometric modelling of the formation and migration of the spot on BY Dra also indicates a differential rotation amounting to -9.7 x 10 to the -9th rad/sec per deg, in agreement with the solar value.

Survey of the BY Draconis syndrome among dMe stars

Results are reported for a BVr photometric survey of 22 dK, dKe, dM, and dMe stars conducted to search for slow quasi-sinusoidal fluctuations in V (the BY Draconis syndrome). The (B-V) and (V-r) color indices are determined in an attempt to detect wavelength-dependent color changes produced by starspots and to infer starspot temperatures. It is found that nine of the stars exhibit variations in V of the order of 0.05 to 0.10 magnitude on a time scale of days or weeks, that at least three more display changes in mean light level over a period of years, that the stars generally tend to become redder at minimum light, and that some of the stars show no detectable color changes over their photometric cycle. The color data are taken to suggest a probable temperature difference of about 200 to 500 K between the stellar photospheres and starspots if the V variations are attributed to dark spots. It is concluded that the BY Draconis syndrome is clearly a very common occurrence among dMe stars.