Spotted Surface Structure of the Active Giant PZ Mon

Abstract

Based on our photometric observations in 2015-2016 and archival photometric data for the active red giant PZ Mon, we have found the main characteristics of the stellar surface: the unspotted surface temperature T e f f =4730 K, the spot temperature T s =3500 K, and the relative spot area from 30 to 40%. The best agreement with the observations has been achieved in our three-spot model including a cool polar spot with a temperature of ∼3500 K as well as large and small warm spots with a temperature of ∼4500 K. The stable polar spot is responsible for the long-period brightness variations. Its presence is confirmed by an analysis of the TiO 7054 Å molecular band. The small-amplitude 34-day variability is attributable to the warm spots located on the side of the secondary component, which determine the relatively stable active longitude.

Cool Spots on the Surface of the Active Giant PZ Mon

“…4. The mean effective temperature T e f f =4717±45 K computed using 2 http://wwwuser.oats.inaf.it/castelli/grids.html all colors is in good agreement with T e f f =4700±100 K, the temperature determined by analyzing spectral lines , and T e f f =4730±50 K derived from our 2016-2017 photometric data (Pakhomov et al 2018). In what follows, we adopt T e f f =4730 K. When computing the magnitudes (eq.1), the effects of interstellar extinction and cool spots are similar.…”

“…This model describes well the spectral energy distribution for PZ Mon computed, according to Bessell et al (1998), from the fluxes in individual bands (see Fig. 6, where the first point corresponding to the U band was taken from Pakhomov et al (2018)).…”

“…On average, the values of the B−V color itself lie in the range from 1.16 m to 1.18 m , which has been observed already for a long time even at considerable brightness V differences from 9.0 m to 9.5 m . These observational facts can be interpreted as the presence of a cool region with a considerable area in the stellar photosphere (Pakhomov et al 2018). Indeed, a surface considerably cooler than the quiet photosphere emits much less energy in the visible range, for example, in the B band: F(4730K)/F(3500K) ≈ 20; therefore, its presence is not revealed by an analysis of the short-wavelength part of the optical range.…”

“…We adopted E(B−V)=0.06 mag, R X = 4.08, 3.10, 2.58, 1.85, 0.88, 0.56, 0.34, and 0.28 for the BV RI JHK L bands, respectively. Pakhomov et al (2018) estimated the cool spot temperature T s =3500 K by analyzing the TiO molecular band. It can also be seen from Fig.…”

“…We constructed a model for the intensity distribution over the surface of the star under study bytechnique similar to that in Pakhomov et al (2018). On a 720×360 map there are a cool polar spot with a radius of 63.7 • (this provides 41% of the visible surface, given the limb-darkening effect) and a warm equatorial region that is represented by a slightly different model in this paper.…”