Near-infrared evidence for a sudden temperature increase in Eta Carinae

Abstract

Aims. Eta Car's ultra-violet, optical, and X-ray light curves and its spectrum suggest a physical change in its stellar wind over the last decade. It was proposed that the mass-loss rate decreased by a factor of about 2 in the last 15 years. We complement these recent results by investigating the past evolution and the current state of η Car in the near-infrared (IR). Results. The long-term brightening trends in η Car's JHKL light curves were discontinuous around the 1998 periastron passage. After 1998, the star shows excess emission above the extrapolated trend from earlier dates, foremost in J and H, and the blueward, cyclical progression in its near-IR colors is accelerated. The near-IR color evolution is strongly correlated with the periastron passages. After correcting for the secular trend we find that the color evolution matches an apparent increase in blackbody temperature of an optically thick near-IR emitting plasma component from about 3500 to 6000 K over the last 20 years. Conclusions. We suggest that the changing near-IR emission may be caused by variability in optically thick bremsstrahlung emission. Periastron passages play a key role in the observed excess near-IR emission after 1998 and the long-term color evolution. We thus propose as a hypothesis that angular momentum transfer (via tidal acceleration) during periastron passages leads to sudden changes in η Car's atmosphere resulting in a long-term decrease in the mass-loss rate.

Near IR observations ofηCar: Reaching its critical rotation?

“…We report a preliminary result in the monitoring of η Carinae in JHKs bands through the "spectroscopic" event of 2014.5 at InfraRed Survey Facility (IRSF) located in South African Astronomical Observatory (SAAO). The latest photometric data, combined with the data taken with the MK-II photometer [1], show a sign of the same cyclic variation in the J-H vs. H-K diagram reported by Mehner et al [2]. The change can still be attributed to an apparent increase in black-body temperature, potentially reaching 3,000 to 6,000K as of March 2015.…”

“…Near-Infrared monitoring of η Carinae continued throughout the spectroscopic event of 2014.5 (for those who are not familiar with the star and its 5.5yr cyclic and other secular changes, please see [2] and references therein). As the last event occurred in August 2014, some observations were proven difficult to conduct (η Carinae being too close to the Sun for many observatories), though we managed to obtain a few crucial data points during the phase of expected rapid changes in JHK colors.…”

Eta Carinae’s 2014.6 spectroscopic event: Clues to the long-term recovery from its Great Eruption

“…Periastron most likely occurs within the range t ≈ −15 to +15 days. This is the same timing system used by Mehner et al (2011bMehner et al ( , 2014. Quoted wavelengths are air values and Doppler velocities are heliocentric.…”

“…We reported the differences between the 2003 and 2009 events and their implications in Mehner et al (2011b). Periastron passages may be triggering the long-term changes observed in η Car's light curves and spectra over the past two decades (Mehner et al 2014).…”

Recovery From Giant Eruptions in Very Massive Stars

“…Davidson (2012) commented on possible time scales for the subsequent recovery. For example, a change of state in the mass-loss rate is currently observed in η Car (Davidson et al 2005;Mehner et al 2012Mehner et al , 2014Mehner et al , 2015, suggesting that the recovery time is in the order of centuries, and the trend has been remarkably unsteady (Humphreys et al 2008).…”