1 / 98

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

“... OK, suppose we do an experiment and it confirms theory. What have we learned? Nothing! Progress comes from experiments where theory fails .” -- mid-20 th century physicists’ truism. An Astrophysical Counter-Paradigm: * The Hubble Treasury Project for Eta Carinae.

melvina-hartnett
Download Presentation

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

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. 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

  2. kd 2005 June 8 -- talk at STScI 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.)

  3. kd 2005 June 8 -- talk at STScI FTRUTH-IN-ADVERTISING DISCLAIMERE This talk shouldn’t be considered “a report on thehCar Treasury Program”, exactly -- because ...

  4. kd 2005 June 8 -- talk at STScI 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. )

  5. kd 2005 June 8 -- talk at STScI ...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 ... 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.)

  6. kd 2005 June 8 -- talk at STScI NGC 3372 , a big star-formation region 7500 lightyears away.

  7. kd 2005 June 8 -- talk at STScI AcloserviewofNGC3372(nowNorthisatthetop)

  8. kd 2005 June 8 -- talk at STScI Eta Carinae and its famous ejecta Strictly speaking, h 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 Msun or 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 h Car itself.

  9. kd 2005 June 8 -- talk at STScI 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 ...

  10. kd 2005 June 8 -- talk at STScI h Car’s pages in the Guinness book 1. Most luminous star that we’re sure about: Certainly L > 3 millionLsun , and probably L» 5 millionLsun . -- Which implies it’s the most massive, too: Initial mass » 140to200 Msun , Present-day mass » 110to150 Msun . 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.

  11. kd 2005 June 8 -- talk at STScI 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 Msunejected, total energy > 1049 ergs) 6. The only naked-eye star that we really don’t understand.

  12. kd 2005 June 8 -- talk at STScI ST.JAMES’SGATE, 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.

  13. kd 2005 June 8 -- talk at STScI 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.

  14. kd 2005 June 8 -- talk at STScI 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 Msun , than we have today. They helped to re-ionize the universe, with various consequences. Two or three years ago, when questioned closely, enthusiasts typically replied, “Very massive stars are straightforward. Simple Thomson-scattering opacity, no convection in the outer layers, nothing to worry about.What could possibly go wrong?” Alas, most of them didn’t know the following history.

  15. kd 2005 June 8 -- talk at STScI THE H-R DIAGRAM HAS A SLOPING EMPIRICAL UPPER LIMIT – NO YELLOW, ORANGE, RED SUPER-SUPERGIANTS. WHY?

  16. kd 2005 June 8 -- talk at STScI LIKELY EXPLANATION: ERUPTIVE MASS LOSS -- A COMPLICATED SURFACE INSTABILITY WHICH ARISES NEAR THE EDDINGTON LIMIT.

  17. kd 2005 June 8 -- talk at STScI 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)

  18. kd 2005 June 8 -- talk at STScI (Supernova Impostors, continued:) The two classic examples

  19. kd 2005 June 8 -- talk at STScI Other Supernova Impostors are also known. A research group interested inSN 1954Jis reportedly planning a TV series on Impostors:

  20. kd 2005 June 8 -- talk at STScI 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!

  21. kd 2005 June 8 -- talk at STScI ETA CARINAE APPEARS TO BE THE MOST EXTREME OBSERVABLE EXAMPLE. INDEED IT INSPIRED THE BASIC IDEA.

  22. kd 2005 June 8 -- talk at STScI 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.

  23. kd 2005 June 8 -- talk at STScI 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.

  24. kd 2005 June 8 -- talk at STScI A third way to begin (even if it’s almost a digression): The GRB Connection – Cosmic Gamma-Ray Bursters.

  25. kd 2005 June 8 -- talk at STScI The connection seems pretty obvious, given that Eta’s mass is very large. And this is one star whose rotation axis we know! * * ( Probably. )

  26. kd 2005 June 8 -- talk at STScI ... Which brings us to a related point about rotation and stellar winds – an important recent observational + theoretical development, in which HST/STIS observations ofh Car played a unique, unconventional, and absolutely essential role.

  27. kd 2005 June 8 -- talk at STScI We can observe the spectrum from the polar direction, via reflection by dust. It’s different in a very significant way! Logarithmic H-alpha profiles (see N.Smith’s PhD thesis)

  28. kd 2005 June 8 -- talk at STScI --- Latitude about 40° Probable interpretation: The wind is densest at the poles, even though this seems counter-intuitive. (Stan Owocki has a fairly logical explanation.) --- Approx. polar view

  29. kd 2005 June 8 -- talk at STScI The photosphere (which is located in the wind) is most likely prolate, with a range of temperatures.

  30. kd 2005 June 8 -- talk at STScI Stellar structure: instabilities, evolution, nature of the Eddington limit Stellar 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) 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.

  31. kd 2005 June 8 -- talk at STScI 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 almost imagine that the STIS was designed specifically to observe Eta Carinae. It wasn’t, but it turned out almost that way.

  32. kd 2005 June 8 -- talk at STScI Ground-based spatial resolution -- even 0.5² -- is simply not adequate for this object.

  33. kd 2005 June 8 -- talk at STScI WITH HST, A DIVERSITY OF TYPES OF SPECTRA, EACH UNIQUE AMONG KNOWN OBJECTS:

  34. kd 2005 June 8 -- talk at STScI

  35. kd 2005 June 8 -- talk at STScI 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 highest angular 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 0.1² FOS aperture. ( continued on next page )

  36. kd 2005 June 8 -- talk at STScI · 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 )

  37. kd 2005 June 8 -- talk at STScI · 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 thatthe 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.

  38. kd 2005 June 8 -- talk at STScI Digression: Pop culture, HST, and explosions in space 1977: In the original version ofStar 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?

  39. kd 2005 June 8 -- talk at STScI 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)

  40. kd 2005 June 8 -- talk at STScI THE CLEAREST EARLY DESCRIPTION OF A SPECTROSCOPIC EVENT IN ETA ...

  41. kd 2005 June 8 -- talk at STScI THE CLEAREST EARLY DESCRIPTION OF A SPECTROSCOPIC EVENT IN ETA ...

  42. kd 2005 June 8 -- talk at STScI ...And later Viotti noticed that the strange 2507 Å emission had also disappeared for a while in 1981 (IUE data)

  43. kd 2005 June 8 -- talk at STScI 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.

  44. kd 2005 June 8 -- talk at STScI 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. )

  45. kd 2005 June 8 -- talk at STScI DOUBLE SIGNIFICANCE OF THE 5.5-YEAR SPECTROSCOPIC CYCLE 1. The periodicity itself may be a clue. 2. --And, given the 5.5-year period, for the first time we knew when such an event would occur!

  46. kd 2005 June 8 -- talk at STScI A binary companion is the most obvious “explanation” for the periodicity. Its orbit must be quite eccentric, though.

  47. kd 2005 June 8 -- talk at STScI Results of the 1997-98 event 1. 2-10 keV X-rays increased tremulously, then crashed. 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 Ha ; etc. Practically no temporal coverage, though, and obs were late in the event.

  48. kd 2005 June 8 -- talk at STScI 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 !

  49. kd 2005 June 8 -- talk at STScI 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.

  50. kd 2005 June 8 -- talk at STScI (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.

More Related