## A primary node in the Epsilon Aurigae Eclipse Campaign

You can get the latest light curves by going to Jeff Hopkin's campaign website:  Campaign newsletter link - which welcomes reports from all serious observers, and importantly CitizenSky where you also can report your visual and digital observations.  For breaking news, any google/bing type searches will turn up stories from the Jan 2010 AAS meeting where several papers were presented, watch the Twitter and  Facebook sites.   Related images can be found at my additional webpage: http://portfolio.du.edu/rstencel - under the epsilon Aur tab.

2015 - With regrets, we announce the passing of Jeff Hopkins, a giant among not-so-amateur astronomers. Safe travels through the universe, Jeff.

More evidence accumulating that epsilon Aur is not as distant as some think, but closer to the 750 pc value established in Brian K's thesis.

### Special issue of the Journal of the Amer. Assoc. Variable Star Observers (JAAVSO Vol.40, #2) includes many reports about epsilon Aurigae—including:     An Overview of the 2009–2011 Eclipse Campaign Results = http://adsabs.harvard.edu/abs/2012JAVSO..40..618S .

+Interferometry of ɛ Aurigae: Characterization of the Asymmetric Eclipsing Disk, Kloppenborg, Stencel, et al. - http://adsabs.harvard.edu/abs/2015ApJS..220...14K
+Transient Carbon Monoxide (CO) Absorption and Persistent Brackett Alpha (Brα) Emission in the Spectrum of ɛ Aurigae, Stencel, Blatherwick & Geballe - http://adsabs.harvard.edu/abs/2015AJ....149..109S
+Constraints from Asymmetric Heating: Investigating the Epsilon Aurigae Disk, Pearson & Stencel - http://adsabs.harvard.edu/abs/2015ApJ...798...11P
+Time-series high-resolution spectroscopy and photometry of ɛ Aurigae from 2006-2013, Strassmeier, Schanne et al. - http://adsabs.harvard.edu/abs/2014AN....335..904S

+Merging Recent and Historic Spectra of ɛ Aurigae: Properties of the System's Components, and Discovery of a Mass Transfer Stream, http://adsabs.harvard.edu/abs/2013PASP..125..775G

+A high angular and spectral resolution view into the hidden companion of ɛ Aurigae, http://adsabs.harvard.edu/abs/2012A%26A...544A..91M

Dec. 2011:

TEN THINGS WE'VE LEARNED AS A RESULT OF THESE CAMPAIGNS - some of these were recognized last time around, and some represent working hypotheses that serve as tools to design more clever measurements and theory to finally resolve the masses and evolutionary status of epsilon Aurigae components:
FACTS:  the eclipsing object is a large, 550K disk [IR];
FACT/INTERPRETATION: neutral potassium monitoring shows disk substructure [LHIRES];
FACT: carbon monoxide re-appeared again after mid-eclipse [SpeX, GNIRS];
NEW FACT: He I 10830A absorption strengthened after mid-eclipse [SpeX];
FACT: the disk facing the F star is heated to 1100K [IR];
FACT: the Far-UV output is somewhat eclipsed [HST/COS];
FACT: no 10 micron silicates [BASS, MIRAC] - INTERPRETATION: large particles ( greater than 1 micron size) dominate the disk;
FACT/INTERPRETATION: light curves feature 0.1 mag variations --> F star oscillations, wind.
SUSPECTED: the disk may contain a B5V star;
SUSPECTED: the mass ratio favors the B star as the more massive object.

Some of these new and old facts lead me to the following considerations.  One can compute the equilibrium temperature of an object orbiting a star [see, e.g. http://burro.cwru.edu/Academics/Astr221/SolarSys/equiltemp.html ].  In a blackbody approximation, for a 30,000 solar luminosity primary star, we find that the observed heated face of the disk, 1100K, is reached at a separation of 9 to 11AU, for a range of particle albedos, from 0.3 (somewhat reflecting) to zero (fully absorbing).  Given that the F star luminosity estimate increases with larger distances, this separation result is not definitive, but these thermal IR facts provide another constraint on binary separation (hence, total mass).   Another interesting constraint on the mass of the central star inside the disk is derived from the velocity shift seen in Robin Leadbeater's neutral potassium line data, approx +/- 35 km/sec rotation speed, plus a current best guess for the radius of the disk itself, 3.8AU.  Kepler's third law then tells us the central mass is 3.85 solar masses - a bit below the 6 solar mass B star proposed earlier.  Watch this space for developments in coming months.

Thus, although the recent eclipse is fading into memory, the bonanza of data is providing researchers both ample constraints for checking the current model, and inspiration for how to design observations that can confirm ideas without waiting another 27 years for the next eclipse.  Key among the goals in these studies is pinpointing the disk’s age and evolutionary state, and whether there might be high levels of activity such as the B star’s accretion of disk matter.  The F star itself is an important part of the study:  Does it have an active atmosphere or giant convective cells, flares, or even a strong stellar wind?  The next eclipse is forecast to start in  2036, but you can enjoy out-of-eclipse variations of Epsilon Aurigae’s light the very next time you see the star, along with Capella and the Kids riding across the evening sky.  Post-eclipse observations are still needed — this star retains its capacity to surprise.   Thanks again for your interest and participation in this campaign, and keep in touch!
--Dr. Bob Stencel, University of Denver Astronomy, rstencel@du.edu

Recent papers: Infrared Studies of Epsilon Aurigae in Eclipse 2011 Nov Astronomical Journal, vol 142, pp.174-183.

plus an interesting olde analysis by Handbury & Williams, 1976:

2010 December:

The Seattle AAS meeting in January 2011 featured a set of contributed posters plus 6 invited talks, all highlighting developments associated with studies of the current eclipse of epsilon Aurigae.  The posters submitted include:

* * * * * * * *

NEWS * 2010 May:

A summary of progress during the first half of eclipse can be found at these websites:

http://xxx.lanl.gov/ftp/arxiv/papers/1005/1005.3738.pdf - "Epsilon Aurigae in Total Eclipse, 2010 - Mid eclipse report"  and

http://www.hposoft.com/EAur09/EAUR%20pdfs/SAS2010.PDF - "Epsilon Aurigae Eclipse 2009 - Ingress Photometry"

2010 April:

A trifecta of new reports are beginning to reveal unprecedented details about the nature of the epsilon Aurigae system:

Development #1 - The complete spectral energy distribution: http://arxiv.org/PS_cache/arxiv/pdf/1003/1003.3694v1.pdf

The observed spectral energy distribution can be reproduced using a three component model consisting of a 2.2+0.9/-0.8 Msun F type post-asymptotic giant branch star, and a 5.9+/-0.8 Msun B5+/-1 type main sequence star that is surrounded by a geometrically thick, but partially transparent, disk of gas and dust. At the nominal HIPPARCOS parallax distance of 625 pc, the model normalization yields a radius of 135+/-5 Rsun for the F star, consistent with published interferometric observations. The dusty disk is constrained to be viewed at an inclination of i > 87 deg, and has effective temperature of 550+/-50 K with an outer radius of 3.8 AU and a thickness of 0.95 AU.

Development #2 - Interferometric imaging of the disk during eclipse ingress: To appear in Nature, 8 April issue.

Synopsis: Interferometric images obtained during autumn 2009 show the opaque disk crossing the southern hemisphere of the F star.  The measured change over the course of two observations a month apart indicate relative motion, which in combination with well-known orbit of the F star, argue for a mass ratio of 0.6 - which means the F star is lighter than the disk and its contents.  Disk opacity can be used to estimate the dust mass of the disk, to be less than one earth mass.  Details available on request from this author.

Development #3 - Substructure inside the disk: http://arxiv.org/ftp/arxiv/papers/1003/1003.3617.pdf

Variations in the equivalent width of the neutral potassium line at 7699A are reported, during ingress and into totality of the current eclipse of the enigmatic eclipsing binary epsilon Aurigae. The increase and plateaus of line strength are correlated with new system parameters and interferometric imaging constraints, plus ancillary data being reported contemporaneously. Together, these data reveal structural details of the transiting disc, never before measured.

2010 Jan: A great deal is being learned during this eclipse cycle (2009-2011) and almost too much to keep this humble webpage up-to-date.  You can get the latest by going to Jeff Hopkin's campaign website:  Campaign newsletter link - which welcomes reports from all serious observers.  For breaking news, any google/bing type searches will turn up stories from the Jan 2010 AAS meeting where several papers were presented, watch the Twitter site, Facebook, and importantly CitizenSky where you can report your visual observations.  Feel free to contact me directly for the latest as well.

See Sky&Telescope article: http://www.skyandtelescope.com/news/80730537.html

BACKGROUND:

An F supergiant star (or post-AGB star) with an enormous grey disk orbiting each 27 years... eclipses last nearly 2 years! Current one started 2009 mid-August and lasts til late spring 2011!   Where to look!

What are the science goals, and what measurements are crucial?  Partial phases of the eclipse have been getting shorter during the 20th century, and the low amplitude out of eclipse light variations have been getting faster during the past 50 years.  In the context of the prevailing model, where a massive disk eclipses the F supergiant star, these changes can be interpreted as disk evolution, possibly due to planet forming activity.  Thus, testing this idea requires good photometric coverage, ideally UBVRI and JHK.  Also, the most sensitive indicators of the disk have been the optical spectra (4000-4500AA and 7699AA).  If the disk is changing, then the blue region and the K I lines should differ from the past eclipse behavior.  Finally, modern interferometry has the chance to directly resolve the disk transiting the supergiant star, and those observations are underway, with an initial report recently published in the Dec. 20, 2008 ApJ Letters.

To join in the campaign effort, send email to phxjeff at hposoft  and/or rstencel at du.edu -- note added 2015: hposoft e-address no longer available

CALENDAR OF ECLIPSE EVENTS:
2009 Aug 6th - predicted start of eclipse, rising with sun as a morning star
2009 Nov/Dec - eps Aur transits at midnight --> evening star
2009 Dec 21 - predicted start of totality, evening star
2010 May - eps Aur setting with the sun, mid eclipse brightening?
2010 Aug 1st - predicted time of mid-eclipse, early morning star
2010 autumn - end of mid-eclipse brightening?
2010 Nov/Dec - eps Aur transits at midnight --> evening star
2011 March 12 - predicted end of totality, evening star
2011 May 15 - predicted end of eclipse, next one starts in 2036.

# *Interferometry at PTI, ApJ Letter [pdf link].* Latest photometry report, Hopkins et al. 2008 [pdf].** SAS interferometry report, Mais et al. 2008 [pdf]. *** Link to the latest Campaign Newsletters! [webpage] *Join the IYA Citizen Science effort [link].

## TOPICS: Background; Goals; References; Links to News, and more!

### GOALS: The science goals for observing epsilon Aur are several: [1] photometry: help determine whether the ~0.1 mag quasi-periodic, out of eclipse light variation is due to the F supergiant star, or related to excitation of disk material (UV and infrared spectra); and, have that variation determined well enough to constrain whether the mid-eclipse brightening seen previously is merely F supergiant variation or could be a central clearing in an inclined disk; [2] spectroscopy: a �disk trailing wake� appears to influence the light curve and spectra only after mid-eclipse (Canavaggia 1980, Ferluga and Mangiacapra, 1991), but pre-eclipse observations are needed to determine whether material is symmetrically distributed about the disk along the orbit; [3] directly test the Huang disk model interferometrically, to witness whether the single F supergiant stellar disk (2.1 milli-arcseconds measured diameter) does bifurcate into a pseudo-binary during eclipse due to the dark disk superpositioning; [4] confirm the polarimetry results obtained during the previous 1984 eclipse (Kemp et al. 1986). References below. Viscosity-dependent model for twisted accretion disk surrounding putative central binary in the secondary of epsilon Aur (Kumar, 1987).

NEW REPORTS:
A note on wide angle photometry: Jeff Hopkins reports in Campaign Newsletter #3 that--
At the Hopkins Phoenix Observatory we have been experimenting using a 50 mm camera lens with a DSI Pro CCD camera to do BVRI photometry of epsilon Aurigae. Even using the 50 mm lens requires the lens to be stopped down to F.4.0 to keep the R band data in the linear region of the detector. We found that using the 50 mm lens at F/4.0 with 5.7 second exposures for the BV and I bands and 2.0 second exposures for the R band worked well. Each image is a composite of 20 images stacked. Even at only 2.0 seconds the R band data peak ADU counts were over 36,000. Reduced data showed the data spread for 3 set of images for each filter to have a standard deviation of close to 0.01 magnitude. More data will be taken, but so far this looks like an excellent means to do CCD BVRI photometry of epsilon Aurigae.
IYA/US node adopts eps Aur for Citizen Science campaign
Epsilon Aur is AAVSO's variable star of the season!
Report on recent photometry [Hopkins et al. 2006]
Campaign plans for the 2009-2011 eclipse [Lucas, Hopkins, Stencel 2006]
Latest photometry from HPO
Jeff Hopkin's eps Aur compendium page!
List of campaign registrants
PDF of NASA conference publication #2384: The 1982-84 Eclipse of Epsilon Aurigae" (5Mb) HERE.

## Background

F supergiant plus ??? with disk in orbital period of 9885 days (27.1 yr)
VizieR/SIMBAD list P=9892d and epoch (for MAX light?) JD=2,435,629
Distance = 578 +/- 51 pc, Mv = -6.74 +/-0.3
Eclipse durations: 1st-4th contacts 647 days, 2nd-3rd (total) 446 days.

### PREDICTIONS for contacts: Julian Date

1st - partial phase begins 2455050 = 6 AUG 2009
2nd - totality begins 2455187 = 21 Dec '09
mid-eclipse - 2455410 = 01 AUG '10 (**2455394 = 16 Jul '10)
3rd - totality ends - 2455633 = 12 MAR '11
4th - partial phase - 2455697 = 15 MAY '11

OBSERVED contacts, 1982-84 eclipse event, per Paul Schmidtke(1985*); J.Hopkins (1990**):
1st - 1982 Jul 14 = JD 2445165
2nd - 1982 Nov 28 = JD 2445302
(mid - JD 2445525)
3rd - 1984 Feb 17 = JD 2445748
4th - 1984 Apr 21 = JD 2445812

*For a free copy of the NASA conference publication #2384, "The 1982-84 Eclipse of Epsilon Aurigae", please email rstencel@du.edu, and provide your complete airmail address. You can now obtain the same in PDF format HERE. Caution - it is a largish file (5Mb).

RECENT OBSERVATIONS:
Fortuntely the approach of the 2009 eclipse has begun to engender community interest in additional precursor observations, especially in the infrared. So far, these include mid IR spectra [Spitzer IRS/MIPS, autumn 2005 and spring 2006]; near infrared spectra [Mimir/Lowell, Dan Clemens Jan.2006]; mid-infrared photometry [IRTF/Mauna Kea, Glenn Orton March 2007] and optical interferometry [NPOI/Flagstaff, Chris Tycner, Mar.2007; PTI/Paolmar, Bob Stencel et al. Oct-Dec.2007]. The spectra and photometry are helping to confirm whether the second component in the system shows evidence of its characteristics, and the optical photometry directly measures an 0.002 arcsecond diameter for the F supergiant. Details of these results will be presented elsewhere.

Intrerferometry finally can provide a direct test of the Huang model for the eclipse - in that the 2.2 milliarcsec F star disk should be bisected by the dark disk and appear as a "double star" with similar separation.

"H-alpha spectra being regularly acquired by Lothar Schanne (reference IBVS 5747) provide a roadmap of the nebular emission from the binary system that enable another dimension to be added to the revelations of the UBVJH photometry reported by Hopkins. The line center velocity gives a Doppler measurement of the densest material, while the emission bump variation informs us of the movement of lower density clouds associated with one of the components. Once again, having this pre-eclipse record will help place in-eclipse variation into a useful context that largely was absent during the run up to the 1982 eclipse."

 Interestingly, the recent V band peak just prior to MJD 54100 does
seem to be flanked by shallow minima about 54050 and 54140... of course
the brightening overall is dramatic and changes are bigger toward the
blue/uv:
MJD      V band changes        B band changes       U band       H band
3000     min 3.12, max 3.04    mn 3.72, mx 3.60   3.85, 3.60     --
4000     min 3.05, max 3.00    mn 3.62, mx 3.55   3.75, 3.65     1.50
diffs         .07       .04        .10      .05    .10   .05     --


Spitzer observations obtained:
eps aur 5: 1:58.13 43:49:23.90 Stencel EPSAUR 20058 mipsphot 21.10 2005-09-25 15:00:11.3 13849088 epsaur-mips1
eps aur 5: 1:58.13 43:49:23.90 Stencel EPSAUR 20058 mipssed 32.80 2005-09-25 15:18:02.2 13849344 epsaur-mipsed1
eps aur 5: 1:58.13 43:49:23.90 Stencel EPSAUR 20058 irsstare 10.48 2005-10-19 07:40:03.7 13848832 epsaur-irs1

**Hopkins, J. 1990 3rd ed. "Zen and the art of photoelectric photometry" uses epoch 2,435,624 and P=9885 days (HPO, 7812 W. Clayton Dr. Phoenix, AZ 85033)

SOME RECENT PAPERS:

2007
2006
Recent UBVJH Photometry of Epsilon Aurigae
Authors: Jeffrey L. Hopkins, Robert E. Stencel
http://arxiv.org/abs/0706.0891
2005
http://www.aas.org/publications/baas/v37n2/aas206/403.htm
[note: The analogy with zeta Aur systems is compelling, but their UV spectrum is darn complicated]
[40.05] To B or not to B: The Companion of Epsilon Aurigae Unveiled P.D. Bennett (CASA, U.Colorado), T.B. Ake (JHU/CSC), G.M. Harper (CASA/ARL, U.Colorado)
Epsilon Aurigae, a bright third-magnitude star in the northern sky, has puzzled astronomers for most of the last century. The optical and ultraviolet spectrum longward of Lyman-\alpha is that of an F~supergiant. It is distinguished by having the longest known period, 27.1 years, of any eclipsing binary. The spectrum implies a massive primary (15--30 M\odot) and the orbital solution implies a comparably massive companion. Although the two-year long primary eclipse is flat-bottomed, which should mean the primary star is totally occulted, no secondary stellar spectrum is seen. Shortward of 1600~�\ and longward of 5~\mum, the eclipse is shallower. The most favored model for this system, which explains the optical light curve and excess infrared flux, is that the secondary object is a large, dark, cold disk (presumably with an optically-obscured star at its center), seen nearly edge-on, which partially occults the F~star during eclipse.
Recent FUSE observations of the far ultraviolet spectrum of \epsilon~Aur show an emission line spectrum and scaled surface flux quite unlike that of the comparable supergiant \alpha~Car, suggesting the presence of a hot component. The rich emission line spectrum is reminiscent of \zeta~Aurigae stars in eclipse. The \zeta~Aur binaries are supergiant stars that eclipse their main-sequence companions. In these binaries, the emission line spectrum seen during totality is produced by scattering of hot star continuum photons in the wind of the cool supergiant. For \epsilon~Aur, the anomalous FUV spectrum and flux suggest a similar formation: continuum photons from a hot companion embedded in the occulting disk are scattered by the wind of the F~supergiant. The presence of scattered photons down to 1050 �\ implies the companion has a spectral type earlier than B5. We use a variety of multi-wavelength observations to constrain the parameters of the newly unveiled B-star in \epsilon~ Aur.

==================

Far Ultraviolet Spectra of eps Aur: Epsilon Aurigae; FUSE Program ID: P135; T. B. Ake
Proposal Abstract
FUSE will be used to study the nature of the unusual eclipsing spectroscopic binary, epsilon Aurigae. The most favored model of this system is that the secondary object is a large, cold disk seen nearly edge-on. IUE and GHRS observations indicate the existence of a far-UV excess compared to other A-F type supergiants, presumably from a hot star in the center of the disk. The main difficulty in interpreting the UV data is that the primary star still contributes significant flux down to 1400-1500 Angstroms. FUSE observations will perform a more direct examination of the secondary, free from contamination by from the photosphere of the primary star. Measurements will be made to determine the physical parameters of the central star, and study variability and gas motions in the disk.
P1350101000 HD31964 05 01 58.13 +43 49 24.0 40 2001-01-07 13:46:00 47715.617 P135 LWRS
Data available at: http://archive.stsci.edu/fuse/index.html

==================

Infrared Photometry of Five Long-Period Binaries, Taranova & Shenavrin 2001 Astron.Let. 27:338
"The 3.5- and 5-micrometer radiation from the eclipsing binary epsilon Aur outside the eclipse exhibits excess (relative to the light from an F supergiant) fluxes which correspond to the emission from a cool source with a temperature of ~1000 K. For the eclipsing binary epsilon Aur, we present the hitherto unpublished results of our optical and IR photometry during 1982-1985, when a primary eclipse was observed in the system."

==================

Intereclipse Spectroscopic Snapshot of epsilon Aurigae with the Hubble Space Telescope, Sheffer & Lambert, 1999 PASP 111:829.
"The spectrum as recorded between 1175 and 1461 A is rich with emission and absorption lines which include stellar and interstellar components. The emission-line profiles have the appearance of double-peaked emission with a stronger red component at a radial velocity of +108 km s^-1, an overlying unresolved absorption component at -20 km s^-1, and a weaker blue emission bump at ~-92 km s^-1. "

==================

Is the eclipsing variable EE CEP a cousin of epsilon Aur?
Mikolajewski, M.; Graczyk, D.
1999 MNRAS 303: 521
We report the first five-colour Johnson UBVRI observations of the last eclipse of the long-period (5.7 yr) eclipsing binary EE Cep. We propose that the star is a member of the Cep OB1 association at a distance of 2.75 kpc. Using this assumption, we find that the primary is a B5 bright giant of radius ~ 10 R_solar and luminosity M_v ~ -3.1. The observations show that the obscuring body is not a star-like object. We suggest that the invisible companion in EE Cep is a dark, thick disc around a low-luminosity central star or binary, and that the system has a few important similarities to the epsilon Aur system.

==================

MORE BIBLIOGRAPHY
Carroll, S. et al. 1991 ApJ 367: 278 - interpreting eps Aur.
Guinan, E. and DeWarf, L. 2002 in Exotic Stars, ASP Conf. 279, p.121.
Ludendorff, H. 1903 Astron.Nachrichten 164: 81.
Stencel, R. 1985 NASA Conf. Publication 2384 - The 1982-84 eps Aur eclipse.
Wright, K.O. 1970 Vistas in Astron. 12: 147 - the zeta Aur stars.

### HISTORICAL NOTES:

At 14:29 -0700 5/3/05, Lucas, Gene wrote:
Hi Jeff,

I just came upon two historic articles on Epsilon Aurigae which I
believe may be of some interest.  These were both published in 1904,
and give visual magnitudes over a span of years back to 1842!!  The
first article, by Col. E.E. Marckwick in the Monthly Notices of the
Royal Astronomical Society (MNRAS) goes over some of the history, and
summarizes his observations back to 1888 (mostly out of eclipse; he
missed the actual eclipse in 1902). Marckwick in turn refers to a
longer 1904 article by Prof. Lundendorff in the Astronomische
Nachrichten (AN)  which summarizes known observations back to 1842!
Marckwick gives his data (with comparison stars and graphs), and
makes the following comment in closing:

"There is a moral in all this for variable star observers.  Do not
tire in watching a variable such as the one now in question, or ?..
One may observe for years without any change, and when one least
expects it an important and marked change may occur.  Although,
according to Lundendorff's result, no further change is due for
twenty-five years [written in 1904; the previous eclipse was in
1901-2 -- GAL], yet I would urge observers to keep a watch on
(epsilon) Aurigae with a view to confirming the remarkable result

Here are the citations.  These are available on the web at the
NASA-ADS web pages, and I have printed out both articles and have
copies for you.  Also, I could send you the files electronically or
provide them on a disk.  The MNRAS article (PDF file) is about 450
kB, and the Ludendorff article is approx. 2.9 Mb.

Marckwick, E.E., "Note on the Variation of (epsilon) Aurigae."  MNRAS
1904, Vol. 65, pp. 83-88.

Lundendorff, H. Von, "Untersuchungen uber den Lictwechsel von
(epsilon) Aurigae."  AN 1904, Vol. 164, No. 3918-19-20, pp.81/82 to
113/114.  (Note that the pages are double numbered, formatted in two