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Register a new userDiscovery of Five Candidate Analogs for $eta,$Carinae in Nearby Galaxies
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The late-stage evolution of very massive stars such as $eta$ Carinae may be dominated by episodic mass ejections which may later lead to Type II superluminous supernova (SLSN-II; e.g., SN 2006gy). However, as long as $eta$ Car is one of a kind, it is nearly impossible to quantitatively evaluate these possibilities. Here we announce the discovery of five objects in the nearby ($sim4-8$ Mpc) massive star-forming galaxies M51, M83, M101 and NGC6946 that have optical through mid-IR photometric properties consistent with the hitherto unique $eta$ Car. The Spitzer mid-IR spectral energy distributions of these $L_{bol}simeq3-8times10^{6} L_odot$ objects rise steeply in the $3.6-8 mu$m bands, then turn over between $8$ and $24 mu$m, indicating the presence of warm ($sim400-600$ K) circumstellar dust. Their optical counterparts in HST images are $sim1.5-2$ dex fainter than their mid-IR peaks and require the presence of $sim5-10 M_odot$ of obscuring material. Our finding implies that the rate of $eta$ Car-like events is a fraction $f=0.094$ ($0.040 < f < 0.21$ at $90%$ confidence) of the core-collapse supernova (ccSN) rate. If there is only one eruption mechanism and SLSN-II are due to ccSN occurring inside these dense shells, then the ejection mechanism is likely associated with the onset of carbon burning ($sim 10^3 - 10^4$ years) which is also consistent with the apparent ages of massive Galactic shells.
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The survey for DUST in Nearby Galaxies with Spitzer (DUSTiNGS) identified several candidate Asymptotic Giant Branch (AGB) stars in nearby dwarf galaxies and showed that dust can form even in very metal-poor systems (Z ~ 0.008 $Z_odot$). Here, we present a follow-up survey with WFC3/IR on the Hubble Space Telescope (HST), using filters that are capable of distinguishing carbon-rich (C-type) stars from oxygen-rich (M-type) stars: F127M, F139M, and F153M. We include six star-forming DUSTiNGS galaxies (NGC 147, IC 10, Pegasus dIrr, Sextans B, Sextans A, and Sag DIG), all more metal-poor than the Magellanic Clouds and spanning 1 dex in metallicity. We double the number of dusty AGB stars known in these galaxies and find that most are carbon rich. We also find 26 dusty M-type stars, mostly in IC 10. Given the large dust excess and tight spatial distribution of these M-type stars, they are most likely on the upper end of the AGB mass range (stars undergoing Hot Bottom Burning). Theoretical models do not predict significant dust production in metal-poor M-type stars, but we see evidence for dust excess around M-type stars even in the most metal-poor galaxies in our sample (12+log(O/H) = 7.26-7.50). The low metallicities and inferred high stellar masses (up to ~10 $M_odot$) suggest that AGB stars can produce dust very early in the evolution of galaxies (~30 Myr after they form), and may contribute significantly to the dust reservoirs seen in high-redshift galaxies.
During the years 1838-1858, the very massive star {eta} Carinae became the prototype supernova impostor: it released nearly as much light as a supernova explosion and shed an impressive amount of mass, but survived as a star.1 Based on a light-echo spectrum of that event, Rest et al.2 conclude that a new physical mechanism is required to explain it, because the gas outflow appears cooler than theoretical expectations. Here we note that (1) theory predicted a substantially lower temperature than they quoted, and (2) their inferred observational value is quite uncertain. Therefore, analyses so far do not reveal any significant contradiction between the observed spectrum and most previous discussions of the Great Eruption and its physics.
We present preliminary results of our analysis on the long-term variations observed in the optical spectrum of the LBV star Eta Carinae. Based on the hydrogen line profiles, we conclude that the physical parameters of the primary star did not change in the last 15 years.
Gaia parallaxes for the star cluster Tr 16 reveal a discrepancy in the oft-quoted distance of Eta Carinae. It is probably more distant and more luminous. Moreover, many presumed members may not belong to Tr 16.
Aims. Eta Cars 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 eta Car in the near-infrared (IR). Methods. We present JHKL photometry of eta Car obtained at SAAO Sutherland from 2004-2013 with the Mk II photometer at the 0.75-m telescope and JHKs photometry with SIRIUS at the 1.4-m IRSF telescope from 2012-2013. The near-IR light curves since 1972 are analyzed. Results. The long-term brightening trends in eta Cars 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 eta Cars atmosphere resulting in a long-term decrease in the mass-loss rate.
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