An Astrophysical Counter-Paradigm: * The Hubble Treasury Project for Eta Carinae

kd 2005 June 8 -- talk at STScI

“... OK, suppose we do an experiment and it confirms theory. What have we learned?

Nothing! Progress comes from experiments where theory fails.”

-- mid-20th century physicists’ truism


An Astrophysical Counter-Paradigm:*The Hubble Treasury Project for Eta Carinae * ( Which is the counter-paradigm? -- Well, mainly I

mean

Eta Carinae; but our project is also somewhat

unconventional.)


TRUTH-IN-ADVERTISING DISCLAIMER This talk shouldn’t be considered

“a report on the Car Treasury Program”, exactly -- because ...


The topic is honestly too large! We only have time for a rather breathless outline of just a fraction of the project.

( More about this later. )


Carina: A constellation in the far south of the

sky; formerly Argo, Argo Navis, or even Robur

Carolinum. (But that last one is another story – history, not

astronomy.)

...The most spectacular spiral arm in our quadrant of the Galaxy lies in that direction.

For example, at galactic longitude 287°, latitude 1°, we can see ...


NGC 3372 , a big star-formation region 7500 lightyears away.


A closer view of NGC 3372 (now North is at the

top)


Eta Carinae and its famous ejecta

Strictly speaking, Car is the

star at the center. The bipolar structure is the “Homunculus Nebula”, ejected in a titanic eruption observed from 1836 to 1860. The ejected mass was 5 Msunor more, but the central star survived. After 160 years of expansion at 650 km/s, the

Homunculus’ polar diameter is currently about 0.2 pc or 0.65 lightyear.

Here, though, we’re mainly interested in Car itself.


This object is uniquely significant for astrophysics in a variety of ways. In fact it really amounts to a surprisingly broad topic, and the hard part of describing it is to choose where to start.For example, we might begin with its impressive list of superlatives ...


Car’s pages in the Guinness book

1. Most luminous star that we’re sure about:

Certainly L > 3 million L sun , and probably

L 5 million L sun .

-- Which implies it’s the most massive, too:

Initial mass 140 to 200 M sun ,

Present-day mass 110 to 150 M sun . 2. Most extreme stellar wind. Mass-loss rate is

around 0.001 M sun per year. That’s 300

to 1000 more than a normal massive star.


Guinness list, continued

3. Brightest extra-solar object in the sky at mid-IR

wavelengths.

4. Probably the strongest, hottest thermal X-ray

source among known stars. (kT ~ 5 keV)

5. Biggest non-terminal stellar explosion that we

know much about. (5—10 M sun ejected, total

energy > 10 49 ergs)

6. The only naked-eye star that we really don’t

understand.


ST. JAMES’S GATE, DUBLIN

...These superlatives are potentially valuable, because often one learns something by exploring the extremes of parameter space.

In particular, that can be a good way to learn which parts of theory don’t work.

The next page shows a real example.


The Eddington Limit is fundamental and ubiquitous in astrophysics -- re. stars, black holes, accretion disks, AGN’s, etc.

A few years ago Nir Shaviv discovered that it doesn’t work the way that everyone assumed. His best, most concrete example was the Great Eruption of Eta Carinae.


A second way to begin: The most massive stars have been fashionable among some cosmologists. (And what could be more fashionable than cosmology?)

According to astrophysical folklore dating back about 40 years– and supported by theory, sort of – the first generation of stars included a larger fraction of very massive objects, M

> 60 M sun , than we have today. They helped to re-ionize

the universe, with various consequences.

Two or three years ago, when questioned closely, enthusiaststypically replied, “Very massive stars are straightforward.Simple Thomson-scattering opacity, no convection in the outerlayers, nothing to worry about. What could possibly go wrong?”

Alas, most of them didn’t know the following history.


THE H-R DIAGRAM HAS A SLOPING EMPIRICAL UPPER LIMIT – NO YELLOW, ORANGE, RED SUPER-SUPERGIANTS. WHY?


LIKELY EXPLANATION: ERUPTIVE MASS LOSS -- A COMPLICATED SURFACE INSTABILITY WHICH ARISES NEAR THE EDDINGTON LIMIT.


In extreme cases, we think, the giant eruption is a SUPERNOVA IMPOSTOR.

This is the modern term for Zwicky’s notorious “Type V Supernova”:

-- Peak brightness lower than a real SN, -- Much longer duration than a SN, -- Total energy radiated as photons is roughly the same as in a SN, -- The star survives! (Most of it, anyway)


(Supernova Impostors, continued:)

The two classic examples


Other Supernova Impostors are also known. A research group interested in SN 1954J is reportedly planning a TV series on Impostors:


At any rate, in terms of physics, Supernova Impostors are more mysterious than real Supernovae.

Unlike SNae, the instability mechanism has not been modeled and is not even known for certain. We certainly can’t read about it in textbooks!


ETA CARINAE APPEARS TO BE THE MOST EXTREME OBSERVABLE EXAMPLE. INDEED IT INSPIRED THE BASIC IDEA.


HERE’S THE POINT (or points):

-- No one predicted this phenomenon, which represented a serious gap in theory.

-- It still does, in fact; even 25 years after the basic idea arose, the eruption mechanism is only vaguely understood. (Turbulence + radiation + rotation: a Combination From Hell.)

-- Eta Car has done a lot of other things that also were not predicted by theorists ! Indeed, theory has a downright poor record for this star.


That’s one reason why we call Eta Car a counter-paradigm.

By the philosopher’s criterion (*quote*),

this object is a good experiment, with implications for several branches of astrophysics – not just stars.

It’s also a sobering & healthy rebuke to theoretical complacency.


A third way to begin (even if it’s almost a digression): The GRB Connection – Cosmic Gamma-Ray Bursters.


The connection seems pretty obvious, given that Eta’s mass is very large. And this is one star whose rotation axis we know !

*

* ( Probably. )


... Which brings us to a related point about rotation and stellar winds – an important recent observational + theoretical development,

in which HST/STIS observations of Car

played a unique, unconventional, and absolutely essential role.


Logarithmic H-alpha profiles

(see N. Smith’s PhD thesis)

We can observe the spectrum from the polar direction, via reflection by dust. It’s different in a very significant way!


Probable interpretation:

The wind is densest at the poles, even though this seems counter-intuitive.

(Stan Owocki has a fairly logical explanation.)

--- Latitude about 40°

--- Approx. polar view


The photosphere (which is located in the wind) is most likely prolate, with a range of temperatures.


5th motivation: A DIVERSITY OF SUBTOPICS, embracing multiple branches of astrophysics.

Gas-dynamics in the ejecta (odd structures)

Etcetera -- this ain’t a complete list.

Stellar structure: instabilities, evolution, nature of the Eddington limitStellar wind physics: extreme parameters. Practically the only stellar wind we can view from various latitudes (reflection)

Exotic nebular excitation processes (unique)

Dust formation in Nitrogen-rich circumstances (dust grains in the Homunculus are known to be highly unusual)


6th motivation (not entirely scientific) :

THIS OBJECT IS EXTRAORDINARILY

WELL - MATCHED TO THE HST.

For instance, we need the highest spatial resolution, but that resolution is attainable -- because Eta is bright.We can “push” the instruments more than most other users try to do.

...Indeed, if we didn’t know better, one would almostimagine that the STIS was designed specifically to observe Eta Carinae. It wasn’t, but it turned out almost that way.


Ground-based spatial resolution -- even 0.5 --is simply not adequate for this object.


WITH HST, A DIVERSITY OF TYPES OF

SPECTRA,

EACH UNIQUE AMONG KNOWN OBJECTS:


Quite honestly, this has been one of the most successful of all HST targets, using most of the available instruments.Since 1991 it has consistently held the record for highestangular resolution of any complex spectroscopy.

· FOS 1991 (pre-COSTAR): First spectrum ever obtained of the star itself; enormous mass-loss rate. Discovered the basic nature of the Weigelt blobs.

· WFC-PC 1991-1993: Revealed that the Homunculus Nebula is bipolar, with a mysterious equatorial skirt.

· WFPC2(PC) 1994-1996: One of the most familiar of all HST color images, quickly became a popular-astronomy icon.

· FOS 1996-1997: One of the very few objects ever observed with the tiny 0.1 FOS aperture.

( continued on next page )


· FOC 1991-94, then GHRS 1996: Showed that the Weigelt blobs were ejected long after the Great Eruption, and are moving at slow speeds that defy intuition.

· FOS 1996-97: Discovered several different and unfamiliar classes of exotic emission spectra throughout the Homunculus, each of them unique among known objects.

· GHRS 1996: Pseudo-laser lines near 2507 Å – again, unique in astrophysics.

· STIS 1998-2000: Discovered the UV “iron curtain” during a spectroscopic event, disproved existing orbital velocity models, revealed many other details of the 1998 event.

· STIS 1998-99 and later 2002-04: The central star has recently brightened at an amazing rate.

( continued on next page )


· WFPC2 1994-2000: Proper motion studies of the Homunculus. Dominant ejection date was 1843, during the Great Eruption.

· STIS 2000: Spectra seen from various latitudes via reflection by dust. Result: The stellar wind is essentially polar -- a fairly revolutionary result for wind theory in general.

· STIS 1998-2001: [ Sr II ] and other weird emission lines.

· STIS 2000: Three-dimensional shape and orientation of the Homunculus. Found that the equatorial debris was ejected in two or more separate events.

· STIS 1998-2000: Discovery of a “Little Homunculus” hidden inside the two large lobes. Apparently ejected 50 years after the Giant Eruption.

· STIS and ACS 2002 et seq.: Numerous Treasury Program results, some of them outlined in this talk.


Digression: Pop culture, HST, and explosions in space

1977: In the original version of Star Wars, the Death Star blew up spherically.

Late 1990’s: Mysteriously, the same explosion had now become bipolar – including an equatorial skirt. Now why do you suppose that happened?


20 YEARS AGO: ANOTHER MYSTERY, OR MAYBE A CLUE TO ETA’S STRUCTURE? “SPECTROSCOPIC EVENTS”

High-excitation emission lines temporarily disappeared at various times –

· 1948 (Gaviola data)

· 1964 (Rodgers & Searle data)

· 1981 (classic Zanella et al. paper)

· 1986 (noticed by various observers)


THE CLEAREST EARLY DESCRIPTION OF A SPECTROSCOPIC EVENT IN ETA ...


...And later Viottinoticed that thestrange 2507 Åemission had also disappearedfor a while in 1981 (IUE data)


THE POINT:

IF WE CAN FIGURE OUT WHAT CAUSES A SPECTROSCOPIC EVENT, IT MAY TELL US SOMETHING ABOUT ETA CAR’S STRUCTURE AND INSTABILITIES.

(In fact, Zanella et al. expressed a promising idea -- more about that later.)

BUT IT’S HARD TO DIAGNOSE SPORADIC, POORLY OBSERVED OCCASIONS.


Anyway: High-excitation, high ionization emission lines temporarily disappeared in 1948, 1964, 1981, 1986.

1992: Another event -- observed (so far as I know) only by Augusto Damineli in Brazil.

1996: Based on the 1992 event, Damineli realized that a recurrence period of 5.5 years would fit all the known instances.*

1997-98: The next event occurred at the predicted time.

* ( Today we know the period is about 5.54 yr. )


DOUBLE SIGNIFICANCE OF

THE

5.5-YEAR SPECTROSCOPIC

CYCLE1. The periodicity itself may be a clue.

2. --And, given the 5.5-year period, for the firsttime we knew when such an event would occur!


A binary companion is the most obvious “explanation” for the periodicity. Its orbit must be quite eccentric, though.


2. A few orbits with STIS showed various changes in the spectrum of the star itself – e.g., the UV “iron curtain”. Strong low-excitation absorption -- Fe II, Ca II, etc. – and temporary P Cyg absorption in H ; etc. Practically no temporal coverage, though, and obs were late in the event.

Results of the 1997-98 event

1. 2-10 keV X-rays increased tremulously, then crashed.


Thus, by 2001, the topic had reached a sort of crisis.

1. We knew about the 5.5-year period.

2. We had obtained some minimal spectroscopy in 1998 using the HST’s STIS instrument.

3. The next predicted “event” would occur in mid-2003.

4. HST data were absolutely necessary.

5. We also recognized that HST spectroscopy will not be possible for the next event after that, in 2008-2009. Nor later !


Idea (2001): A Treasury Project!

Criteria for such a program:

-- Data of broad significance. -- Extensive enough to justify and construct a long-term data archive. -- Similar observations will not be possible after the HST ceases to operate.

This topic matched the criteria perfectly.


(Parenthetical: an unprecedented situation)

No instrument with HST/STIS’s capabilities will be available again for a long time – conceivably a long, long time.

Therefore: Many observations, possible from 1997 to early 2004, will now be impossible for the forseeable future.

But the case of Eta Car is even worse! By the time that some capable new instrument becomes available, perhaps 10 to 30 years from now, this star will have changed.


Hence our project. Main goals & components:

1. A huge set of observations during 2002—2004 with STIS, the Space Telescope Imaging Spectrograph. This is, quite honestly, close to the most intensive spectroscopy that HST will ever do.

2. We are developing better software for reducing and analyzing STIS data. (Badly needed for other users as well.)

3. Supplemental images of Eta Car using two other HST instruments, the WFPC2 and the ACS.


(Project goals & components, continued):

4. Parallel observations using the European Southern Observatory’s “VLT/UVES” instrument in Chile.

5. The whole caboodle will go into a special, highly non-routine Data Archive that astronomers will still be able to use 30 or 40 years from now.


Project participants (CoI’s) – listed in more or less inverted order

CoI’s at other institutions who are mainly interested in using the data:Manuel Bautista (Venezuela) Mike Corcoran (NASA/GSFC)Augusto Damineli (Brazil) Fred Hamann (U. Florida)John Hillier (U. Pittsburgh) Nolan Walborn (STScI)

CoI’s in Germany who have obtained the ESO/VLT data: Otmar Stahl Kerstin Weis

Preparation of the big observing plan, and help with material from STIS IDT: Ted Gull (NASA/GSFC)


Project participants, continued

Initial data reduction and modified software:Kazunori Ishibashi (MIT)

System maintenance & linux sanity:J. T. Olds (U MN)

Data analyses & some dandy discoveries:Michael Koppelman (Clockwork Mpls. & U MN)

Data archive & database planning, etc.:

Roberta M. Humphreys (U MN)


Project participants, continued

Linux building, system maintenance, software,data archive, etc. etc.:

Matt Gray (U MN, later Clockwork)

Data analysis, techniques development, somediscoveries and most of the hard work:

John C. Martin (U MN).


Our web site:

http://etacar.umn.edu

The initial data archive is located there and is beginning to be publicly available. (STScI will have a modified version of the archive.)

http://etacar.umn.edu/


SPECTROGRAPH SLIT


SPECTROGRAPH SLIT

OFFSET FROM STAR


Concerning the amount of data ...

Each complete spectrum covered the wavelength range from

170 nm in the UV to about 1 m in the near-IR. To do this,we needed 30 grating tilts. Eta Car is almost unique in gettingcomplete wavelength coverage with STIS.

Since this object is so bright, we typically obtained more than a dozen independent integrations per orbit. This is highlyunusual, and is the chief reason why our data volume is among the largest obtained in any HST program.

Altogether we have several hundreds of separate wavelengthobservations obtained with an 800 x 800 CCD detector.In addition, far-UV data were obtained with a differenttype of detector.

( explain about special processing )


Ca II and hydrogen lines near 390 nm


COMPLEX STRUCTURE IN UV DATA NEAR 290 nm


Vicinity of H , c. 490 nm


Behavior of the H emission line profile during the 1998—2003 cycle. (our STIS data)


Only a small fraction of the project can be included in this talk. Some of the topics we must skip:

· Improvements in data processing for STIS. Some of our techniques may be helpful for HST users in general.

· Data archive details. The STIS/CCD data are publicly available now in convenient form, other data will follow. (Standard STScI archiving is not very suitable for these unusually intensive observations.)

· ACS imaging data obtained for the Treasury Program. In most respects these are the highest-quality images of the Homunculus so far.

· STIS / MAMA / echelle observations in the UV.

-- continued on next page --


-- more topics not covered here --

· Ground-based VLT / UVES spectroscopy which supplements the HST data. (ESO; O. Stahl & K. Weis)

· Observations of the diffuse ejecta. Today we’ll restrict ourselves to the star itself (or rather, its wind).

· Routine “as expected” results on the star, essential for testing models but not particularly exciting.

· Possible single-star models for the event. Contrary to what some authors have said, the companion star has not, repeat not been confirmed or proven. Every existing observation might in principle be explained with just one star. Since a model of that type seems less likely, however, here we’ll save time by assuming

that Car is a binary system.


The 2003.5 event did occur on schedule.Here are some of theobservables used fortiming it.


First point: Eclipse or mass ejection?

This question strongly affects the basic importance of the 5.5–year cycle. ( explain )


Frankly, the eclipse idea never was very appealing. It was motivated by X-ray analogies (0.01% of the luminosity), its predictions even for the X-rays generally failed, and it was unclear for other parts of the spectrum.


(Parenthetical complaint): This mini-debate would make a good case study of scientific procedure. Normal rules of evidence, predictions, etc., have been perceptibly “bent”.

Just about any outcome is customarily hailed as a great success of the eclipse theory. Sorry if this sounds grumpy, but it’s been true!


... And the alternative is older and far more interesting.

(Please don’t take the word “shell” too literally, though.)


(Imagine what might happen near periastron.)


... Some emission-line velocities (hydrogen, helium)appear to make trouble for the eclipse model.

But new and unexpected evidence has appeared in the He II 4687 emission. This feature was first reported by Damineli last year, but its true brightness,other details, and analysis became evident only withour STIS data.


He II 4687 emission

The peak amount wasterrific compared toexpectations, and thepost-maximum decline was catastrophically fast.


Desperate ansatz to explain the He II emission: Soft X-rays from the shocked gas (region 3)

create He++ in region 4 ...

This sketch is VERY idealized (explain).


... Which presents three technical difficulties:

1. Energy budget requires either an extra mass ejection or else a special trick (see next slide);


Energy-level diagram for He II



1. Energy budget requires either an extra mass ejection or else a special trick;

2. But the special trick requires high gas densities – so we need a mass ejection anyway ...



1. Energy budget requires either an extra mass ejection or else a special trick;

2. But the special trick requires high gas densities – so we need a mass ejection anyway ...

3. And the timing of the peak! (next slide)


The He II emission rose rapidly, after the hard X-rays hadalready declined substantially. Thisdoesn’t seem veryeclipse-like.


Moreover, several other bits of evidence strongly suggest that the orbit is oriented roughly as shown here – so periastron is near quadrature.That’s where the event happens!


Some other unexpected developments may be related to each other.

For instance, the star’sapparent brightnessincreased at an almostalarming rate in 2003.

This is a figure fromMartin & Koppelman’spaper.


We suspect that something basic is changing right now, i.e. in this decade! (explain)


The point: Thermal and rotational recovery timescales following an eruption may possibly produce cycles of various lengths.

(Terrestrial precedent: geysers.)

Moreover, we shouldn’t be very surprised if the star’s surface conditions suddenly change.(Explain: sensitive to parameters.)


If developments continue at this rapid pace –

Within 15 years, the star will appear brighter thanits Homunculus Nebula and will be 4th magnitude.To the unaided eye it will again seem as it did toHalley three centuries ago. And the wind maybecome appreciably weaker.


If developments continue at this rapid pace –

Within 15 years, the star will appear brighter thanits Homunculus Nebula and will be 4th magnitude.To the unaided eye it will again seem as it did toHalley three centuries ago. And the wind maybecome appreciably weaker.

In saying this, however, I’m ignoring the time-honored, repeatedly vindicated, STANDARD ETA CARINAE CARTOON:


... What origin can we ascribe to these sudden flashes and relapses? What conclusions are we to draw as to the comfort or habitability of a system depending for its supply of light and heat on so uncertain a source?

-- J.F.W. Herschel, 1847


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An Astrophysical Counter-Paradigm: * The Hubble Treasury Project for Eta Carinae

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