eta Carinae -- Caught in Transition to the Photometric Minimum

ATel #6368; R. M. Humphreys (Univ. of Minnesota), J. C. Martin (Univ. of Illinois-Springfield), A. Mehner (ESO, Chile), K. Ishibashi (Nagoya Univ., Japan), K. Davidson (Univ. of Minnesota)
on 4 Aug 2014; 23:02 UT
Credential Certification: Roberta Humphreys (roberta@umn.edu)

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Comparison of the near-UV and visual HST/STIS/CCD spectra from U.T. July 13 and July 30 show the primary star’s wind in an interesting transition state that has not been observed before. UV magnitudes measured in the near-UV at 2500A and violet at 3300A on July 13 show the rise in flux before the photometric decline, as expected. The 2500A brightness measured on July 30 shows that the decline had begun, but there is no change in flux at 3300A. So the decline had not yet begun at the longer wavelength. Comparison of the combined near –UV and blue-visual low resolution spectra (STIS/CCD, G230LB and G430L) from July 13 and July 30, show the decline in flux in the near UV on July 30, no change in the Balmer continuum (3000-3500A), and an increase in the visual flux. These observations show a wavelength dependence for the onset of the photometric decline. Some evidence for this dependence is present in the UV light curve from the 2009 minimum, but it is much more obvious in this cycle. In the two previous cycles, 2003.5 and 2009.0, the depth of the minimum showed a strong dependence on wavelength (see Fig. 1 Mehner et al 2011); much deeper in the near-UV than the violet band, and relatively weak in the blue-visual. Due to the presence of strong absorption lines around 2600A, the near-UV wavelength band is much more sensitive to changes in temperature and density. Figures showing the current UV light curve and the spectra are available at http://etacar.umn.edu/download/Eta2014/. In a possibly related observation, the current visual light curves from the La Plata Observatory (http://etacar.fcaglp.unlp.edu.ar/) and the AAVSO show a steep increase in brightness as the star approaches its spectroscopic minimum. Indeed eta Car is the brightest it has been since 1863, after the Great Eruption. In comparison with ground based spectra from the SMARTS/1.5m/Chiron (ATel 6334) and from the VLT/UVES, the He II 4686A emission line was approaching its minimum in the STIS spectrum from July 30. For reasons outlined in Sections 4 and 6 of Mehner et al 2011, it will not be surprising if later stages of the event differ compared to 2009. The "second He II 4686 flash" may occur earlier, beginning within the next 10 days. If so, that should be a leading indicator for the reappearance of hard X-rays. This work is supported by NASA funding from STScI for GO-13377.

HST Treasury Program on eta Carinae