L199, A NEW VARIABLE STAR IN M13

During a routine study of the variable stars of Messier 13 (NGC 6205 = Cl 1639+365) we have discovered the variability of L199 (TYC 2588-1386-2), a red giant star member of the cluster: using the photometric data –in V band– obtained in the 2019 and 2020 campaigns we have verified its variability. Data from the Zwicky Transient Facility (ZTF) and Deras et al. (2019) were utilized to determine its type of variability and period estimation: our data suggest that it is a semiregular red giant, similar to the other variables of this type of the cluster, whose cycles of photometric variation are not identical; we derived a period of ~27 days (although without a regular periodicity) and an amplitude smaller: 0.08 ± 0.03 magnitudes in V band although they may be somewhat larger or smaller. With this discovery the cluster now has 63 variable stars.

Disk-Resolved Photometry of Cometary Nuclei: Results from DIXI and Stardust-NExT

Previous comet flyby missions enabled detailed studies of the photometric properties of several cometary nuclei from disk-resolved images, including 9P/Tempel 1, 19P/Borrelly, and 81P/Wild 2. Two recent missions, DIXI and Stardust-NExT, encountered Comets 103P/Hartley 2 and Tempel 1 respectively, expanding the pool of sampled cometary nuclei in their unique ways: Hartley 2 is a hyperactive comet; Tempel 1 was visited and impacted by the Deep Impact dual-spacecraft during its previous perihelion passage. Photometric modeling shows that the global photometric properties of the nuclei of Hartley 2 and Tempel 1 are similar to those of other cometary nuclei. The photometric variation of the hyperactive nucleus of Hartley 2 is about 15%, similar to that of weakly active comets Tempel 1 and Wild 2. The photometric properties of Tempel 1 measured by NExT suggest little change from those measured by DI. These results, together with the photometric properties of Wild 2 and Borrelly, indicate that the photometric properties of cometary nuclei are independent of the activity level and gross geomorphology of cometary nuclei. Instead, cometary nucleus photometric properties might be determined by its outgassing, which leaves low-albedo deposit on the surface and forms similar photometric texture. The time scale for the photometric alteration on cometary nuclei due to outgassing should be much shorter than the dynamic time scale.

Modeling the magnetosphere of the B2Vp star σ Ori E

This paper presents results obtained from Stokes I and V spectra of the B2Vp star sigma Ori E, observed by both the Narval and ESPaDOnS spectropolarimeters. Using Least-Squares Deconvolution, we investigate the longitudinal magnetic field at the current epoch, including period analysis exploiting current and historical data. σ Ori E is the prototypical helium-strong star that has been shown to harbor a strong magnetic field, as well as a magnetosphere, consisting of two clouds of plasma forced by magnetic and centrifugal forces to co-rotate with the star on its 1.19 day period. The Rigidly Rotating Magnetosphere (RRM) model of Townsend & Owocki (2005) approximately reproduces the observed variations in longitudinal field strength, photometric brightness, Hα emission, and various other observables. There are, however, small discrepancies between the observations and model in the photometric light curve, which we propose arise from inhomogeneous chemical abundances on the star's surface. Using Magnetic Doppler Imaging (MDI), future work will attempt to identify the contributions to the photometric variation due to abundance spots and due to circumstellar material.

Estimation on Metric Spaces with Photometric Variation

Model uncertainty comes in many forms. In this chapterweshall examine extensively the variable photometric model in which the underlying image field I(·) is modelled as an element of a Hilbert space I(·) ∈ H(φ) constructed via basis expansion {φi }. Inference involves both the pose and identity of the objects as well as the photometric intensity itself. This corresponds to making the template random, expanding the deformable template to include both the photometric variations and geometric variations.

Metrics on Photometric and Geometric Deformable Templates

In this chapter the metric structure developed for geometric transformations is extended to define a metric structure on the images which they act. Defined as an orbit under the finite and infinite dimensional groups we establish the necessary properties for measuring distance in the image orbit. We extend the results from not only geometric transformation of the images but also to photometric variation in the images.

The Variability of Sunlike Stars on Decadal Timescales

We have combined observations of about 30 sunlike stars from Mount Wilson, Lowell, and Fairborn Observatories to extend our joint time series from 12 to 17 years. The full range of variation on the decadal timescale has probably now been observed for most of our program stars. Statistical relationships between chromospheric and brightness variability derived earlier by us are confirmed. Young active stars become fainter as their chromospheric Ca II HK emission increases, while older less active stars such as the Sun become brighter as their HK emission increases. The Sun's photometric variation still appears somewhat small in amplitude compared to the other stars in this sample with similar mean chromospheric activity.

Long-term variation in the short-period binary system WR46 (WN3P+?)

We studied the short-period (P ≈ 6–8 hr) binary system WR46 using simultaneous photometry and spectroscopy. We show that the long-term photometric variation is accompanied by changes in the equivalent width of the He II Λ5411 emission line and by an apparent period decrease on the rising branch. Furthermore, the brightening of the object is repeating.

Polarimetric versus photometric variability of Wolf-Rayet star winds

Intensive broad band observations of WR stars by Moffat et al. reveals photometric and polarimetric variability on time-scales of hours, similar to that seen on narrow spectral features. The r.m.s. fractional photometric variation is found to be about 20 times the r.m.s. polarimetric variation whilst simple scattering theory predicts ratios of order unity. The possibility that this discrepancy is due to polarimetric cancellation of the Stokes parameters of light scattered from many dense blobs at random positions (known from spectrometric data to be present) is investigated analytically and by simulations. It is found that factors of 20 reduction in the polarimetric variability cannot be achieved for reasonable numbers of blobs and it is concluded that further polarimetric suppression by substantial blob optical depth and/or intrinsic blob emission must play a role. These results put further constraints on physical conditions in WR wind inhomogeneities.