Welcome TESS followers to our latest news bulletin! This week, we are looking at three recent papers from the archive. Enjoy!
Wolf 359 is one of the closest stars to our Sun (distance of 2.4 pc), and a well-characterized single M-dwarf with a mass of 0.11 MSun and radius of 0.14 RSun. It is a highly-variable star with prominent H-alpha emission and frequent flares. As such, it is a prime target for studying stellar activity in late-type stars and its potential effects on planets around M-dwarfs. Pietras et al. (2023) presented the first analysis of a Sun-like X-class flare on Wolf 359 using data from TESS and XMM-Newton, and studied the physical processes responsible for the event. The target was observed by TESS in Sectors 45 and 46 in short-cadence, and the authors detected a total of 103 flares in the two sectors. The Solar-like flare analyzed here occurred on December 21, 2021, starting at 11:10 UT and ending 12:15 UT, and was also observed by XMM-Newton in the fast mode of the UVM2 filter (bandpass of 182-292 nm). The flare’s time of maximum light was detected at 11:26 UT in the UV bandpass, 11:28 UT in the optical, and 11:33 UT in X-rays. The authors measured a growth time of 24 minutes, followed by 40 minutes of decay time, estimated a total flare energy of 1.1 x 1031 erg, a maximum observed temperature of 17.25 x 106 K, and a maximum emission measure of 3.5 x 1050 cm-3. The differential emission measure calculated by Pietras et al. (2023) shows three distinct components with respective temperatures of about 3 x 106 K, 7 x 106 K, and 16-17 x 106 K. From the helium-like O VII triplet line ratio, the authors determined plasma electron density of 5 x 1010 cm-3, noting that it is more than an order of magnitude lower than the estimated average density of the flare loop, and argued that the discrepancy is likely due to the accumulation time for the spectra. Based on the TESS and XMM-Newton observations, Pietras et al. (2023) conclude that the processes and mechanisms responsible for stellar flares are consistent with those producing solar flares, and that the geometrical properties of the phenomenon are comparable to similar events occurring on the Sun.
Observations of early time lightcurves of Type Ia Supernovae (SNe Ia) are critical for better understanding the inner workings of these spectacular astrophysical phenomena, from the nature of their progenitors to the explosion mechanism and its aftermath. Large-scale photometric surveys can enable such observations and TESS is ideally-suited to monitor a large number of bright Supernovae. Fausnaugh et al. (2023) present an analysis of SNe Ia observed by the mission, aimed at studying the morphology of the early time observations and evaluating potential signatures of companion interactions. The authors performed a visual inspection of 50 Sectors of TESS data from the first four years of the mission, searching for the 850 SNe Ia known to have occurred during this time and identifying 307 lightcurves which exhibited a clear SN Ia rise following a relatively flat pre-explosion baseline. The remaining 543 targets are either too faint to produce the desired signal in TESS, or the explosion occurred before/after the TESS observations. Utilizing comprehensive numerical simulations, Fausnaugh et al. (2023) further constrain the number of targets amenable to reliable measurements of the corresponding power-law index and time of first light to 74 SNe Ia. The rest are excluded from the author’s statistical analysis due to the insufficient signal-to-noise ratio of the TESS lightcurve. For the 74 targets, Fausnaugh et al. (2023) measure an average power-law index of 1.93+/-0.57 and a corresponding average rise time of 15.7+/-3.1 days. The former is consistent with a two-component distribution – a normal distribution with a mean of 2.29 and standard deviation of 0.34, and a long tail with a power-law index smaller than 1. Fausnaugh et al. (2023) argue that this may represent either two underlying SN Ia populations, or a particular range of SN Ia parameters and environmental factors. The average rise time is consistent with a Gaussian distribution with a mean of 15.89 days and a standard deviation of 3.82 days; it is shortest for SN2019gqv (8.4 days) while longest for SN2020bqr (22.9 days). The authors note that the models for SN2020abqu, SN2021ahmz, and SN2022ajw show a slight preference for the presence of a companion, while the models for SN2020tld and SN2022eyw disfavor this scenario. The exquisite photometry from TESS enabled the rare opportunity to study in-depth the early time observations for a large number of Type Ia SNe.
Catalog of Integrated-Light Star Cluster Light Curves in TESS (Wainer et al. 2023) :
Detailed investigations of stellar clusters (both globular and open) enable key insights into star formation and galaxy evolution, and provide vital information on the processes responsible for stellar variability. As resolving all individual members of distant clusters is practically impossible, determining their properties depends on integrated light methods providing reliable measurements with realistic uncertainties. Wainer et al. (2023) present results from an investigation of the TESS lightcurves of stellar clusters containing high-amplitude stellar variables (Cepheid and RR Lyrae stars). The authors developed a new, publicly-available pipeline (https://elk.readthedocs.io/) designed for extracting the integrated light of star clusters in the Milky Way and the Magellanic Clouds, and provide a corresponding lightcurve catalogs of several clusters. Wainer et al. (2023) selected their targets such that the respective cluster is larger than one TESS pixel, its radius is smaller than 0.25 degrees, and the corresponding stellar population is resolved. Altogether, the authors studied 348 clusters – 124 in the Milky Way, 106 in the Small Magellanic Cloud, and 118 in the Large Magellanic Cloud – observed by TESS in Sectors 1-39. Wainer et al. (2023) extracted a total of 2024 per-sector Full-Frame Image integrated lightcurves, and delivered them to MAST as High-Level Science Products. The authors confirmed that the stellar variability of individual stars – both high- and low-amplitude – is preserved in the integrated lightcurves. This highlights the potential of TESS observations to enable detailed astrophysical investigations of stellar clusters in the distant Universe.
Fig. 1: Taken from Pietras et al. (2023). Left panel: X-ray observations of the Dec 21, 2021 Sun-like X-class flare on Wolf 359, along with the corresponding profiles. Right: TESS lightcurve of the flare.
Fig. 2: Taken from Fausnaugh et al. (2023). Lightcurves of example Type Ia SNe observed by TESS, along with the best-fit models as outlined in the figures, and the corresponding residuals.
Fig. 3: Taken from Wainer et al. (2023). Left panel: Integrated TESS lightcurve of the cluster NGC 330. Right panel: Normalized pixel-level flux at the time of the vertical green line in the left panel. The green circle represents the radius of the pixels used to extract the lightcurve.