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epsilon Aurigae - Instant Science from Historic Spectra
epsilon Aurigae has baffled generations of astrophysicists. Although the obvious (dominant) star -- an early-F supergiant -- is eclipsed during its 27.1-year period such that the overall brightness drops by ~1 magnitude in V, it has been widely stated that the spectrum of the system does not alter. Moreover, the object that moves in front must have gigantic proportions. Those facts are what have made epsilon Aurigae traditionally famous. However, it is not actually true to say that the spectrum does not change. It is still recognizeably that of an early-F supergiant, but it changes enough in detail during eclipse ingress and egress to provide invaluable information about the mysterious body that was in front throughout 2010 (and still is,
just). Our concepts of that secondary are still largely speculative, though we have now seen that it is indeed large, cool and elongated in projection.
In order to find out whether that secondary is uniformly opaque, rather than a loosely-bound swarm of particles, we must examine successive eclipses to see whether the same changes repeat. So -- we can sit tight and wait for N X ,27.1 years, or we can attempt `instant science' by examining historic spectra. All historic spectra from the decades we need are photographic; they pre-date the digital era. To access their information quantitatively we have to go back to the originals and convert them into electronic records - something which has never been attempted on this scale.
The technology for digitizing large numbers of these scientific observations to the required level of accuracy is as new as the data are old. Commercial flat-bed scanners are not appropriate -- they were not designed to be - and reliable work requires a purpose-built instrument such as a `PDS' microdensitometer, plus a skilled operator. The PDS at the DAO was recently upgraded to meet the demands of this task (and is one of the very few operational and modernized PDSs in N American astronomy). It was used to digitize about 100 high-resolution photographic spectra of epsilon Aurigae from Mount Wilson Observatory and from the DAO, providing for the first time
electronic versions of eclipse spectra during the 1956 and 1983 events. I could then compare them with over 120 new, high-quality CCD spectra that I recorded with the DAO's 1.2-m telescope during the current eclipse.
Comparisons of spectra at the same orbital phases of those three eclipses demonstrated quite convincingly that the secondary object is stable -- on a time-scale of at least half a century -- and is uniformly opaque. All the spectra, from near-central eclipse as well as from ingress and egress, pointed to that same property. The tiny changes in line-profiles which I was able to
monitor, particularly towards ingress and egress, showed that even in detail the features repeat exactly from one eclipse to another (to within the errors of the data).
That result, though technically a `null', is in fact tremendously helpful. It means that I can piece together the observations from different eclipses, and thus build up a much more comprehensive data-set to represent the whole story.
Hi Robin, Thanks for the suggestion. We found a few, and are scanning them right now (as a special favour ...) but they are not during eclipse - phases .3 to .5, roughly. Elizabeth
Hi Elizabeth, Thanks for looking at 7699A. There is a small component outside eclipse which appears to be interstellar. Results well ouside eclipse will be useful to confirm this. The forum does not seem to handle attachements in a consistent manner unfortunately. It appears as a clickable link in the thread on the website but hopefully this link will also find it http://www.citizensky.org/sites/default/files/epsaur_7699_EW.png You may also recognise this poster from AAS Seattle this January which has more information on amateru measurements of what has been happening spectroscopically this eclipse. www.threehillsobservatory.co.uk/astro/AAS217_poster_257_04_original.pdf The spectrum of the system shows quasi-periodic variationsin RV in many linesoutsideeclipse. Stefanik et al (2010) http://arxiv.org/abs/1001.5011v1 plot them as residuals tothe orbitalsolution for example and the poster shows Buil'smeasurements during eclipse, including instances of line doubling. Are similar variationsseen historically or are they within theerror bars? Best Regards Robin
Hi Robin, We unearthed one particularly good spectrogram, taken at Palomar in Nov 1968 and having a reciprocal dispersion of 13 A/mm and adequate S/N. It has now been scanned, and an electronic version has been prepared. Ihave tried to include the relevant region as an attachment (pdf file) but cannot tell if you are able to view it. The K I wasline is present; there are two weaker oxygen lines just shortward of it. The spectrum is in the rest-frame of the iron-arc reference spectrum, so it contains the velocity of the star and its heliocentric coponent. The computed orbital phase was .55, the RV of the primary -8.5 km/s, and the computed heliocentric correction was +14.5 km/s. The nett result does not at all affect the identification of that small feature near 7698.8 A as K I (7698.98 A). A cavet, however. The spectrum may not have been correctly calibrated in intensity. At Palomar the coude system is on-axis, and requires exposures for intensity calibration to be made on separate pieces of glass. Although the observer will almost certainly have carried out such an exposure, we do not have it here, nor do we have the detailed information about the plate as can (only) be found in the log-book. Both the calibration information and the exposure information entail a visit to Pasadena, which may or may not be wangled into my schedule for next October. It is not easy to guess at all reliably what the calibration conversion could be, since it is a 1-N emulsion and the behaviour of its eventual characteristic curve will depend on the method of hypersensitation that was used - again, only the log-book can tell me that. In the meantime I have used what may prove to be a tolerable approximation to the calibration characteristics, but the equivalent width of the feature in the attached plot could be in error by as much as tens of a percentage in either direction. I am puzzled by your remark that the potassium line is absent out of eclipse, since your plot of its growth during eclipse makes the point that the pre-eclipse component is included. Cheers, Elizabeth
Hi Elizabeth, Thanks for reducing this spectum. Please also see my private message. Yes there is a small outside eclipse component. You can see here for example to theviolet of the eclipsingobjectcomponent which appeared aroundingress . http://www.threehillsobservatory.co.uk/astro/epsaur_KI_18mar-5aug09.png but there is evidence to suggest it is probably interstellar in origin rather than from the eps Aur system. Your spectra may help to confirm this. Cheers Robin
nice post
Do you have any historical data covering the neutral potassium line at 7699A? This line isparticularly interesting as there is no component detected from the eps aur system outside eclipse so the inherent variability of the F star seen elsewhere in the spectrum does not contaminate the result. A strongcomponentappearsduring eclipse (and for a significant period before and after). I have been following the variability in intensity an line profile at high resolution at a high (typically ~4-5 day) cadence and can see significant fine scalevariations and possible deviations from the admitedly rather sparse data produced by Lambert and Sawyer last eclipse. Attached is the current Equivalent Width graph compared with the Lambert and Sawyer data. The line is currently still strong and based on last eclipse should still be detectable until late in the year. Robin