Central Stars of Planetary Nebulae

Grid of synthetic atmosphere models for H-deficient [WCE] CSPN from Graziela Keller 2011, PhD Dissertation, Keller et al. 2011, MNRAS, 418, 705:

Hdef grid A grid of 199 models for H-deficient CSPN, covering a stellar temperature range from 50,000K to 200,000K, with various values of mass-loss rate, transformed stellar radius, and wind velocity. The grid provides line-blanketed spectra with wind from 900Ang to 50,000Ang (continuum spectrum from 300Ang), at varying spectral resolution, computed with the CMFGEN code. The synthetic spectra are available, as well as many diagnostic plots, from Keller et al. (2011, MNRAS, 418, 705)

A new discovery - An old mystery solved: FUSE reveals secret of aging stars

What's the new discovery?
Using NASA's Far Ultraviolet Spectroscopic Explorer (FUSE) satellite, we have determined that a strong ``P Cygni'' line seen in the spectra of hot Central Stars of Planetary Nebulae (CSPN) originates from highly ionized neon in the star's atmosphere. This is an important discovery, because the amount of neon in the star's atmosphere at this phase in the stellar life, is a clue to its past, and future, evolution. The identification of this feature will help us understand the advanced stages of evolution of stars like our Sun, as well as how these stars impact the evolution of the galaxies they inhabit.

How was it discovered ?
A "spectrum" is a graph of how much energy is emitted at each wavelength (different wavelengths correspond to different energy, or "colors"). The first figure shows the FUSE spectrum of a very hot CSPN, NGC 2371. Some of the features seen in a star's spectrum come from ions of different elements in the star's atmosphere (an "ion" is an atom which is missing some of its electrons). These features have a typical profile (called P Cygni profile) when the star has a strong "stellar wind" - i.e., it when it is expelling material from its surface into space. Two such features are seen here - the one on the right is due to OVI which is oxygen with 5 of its electrons missing. We discovered the feature on the left to be caused by NeVII ("neon-seven", or neon missing 6 electrons), and not by CIII, as was widely believed prior to our work. Ascribing this line to CIII did not seem consistent with other clues suggesting that the star is extremely hot, and the transition energy of the CIII line is a bit off the observed transition, so that attributing the line to CIII would also imply huge velocities in the stellar winds, that could hardly be explained.

We use computer programs (called "stellar atmosphere codes"), which simulate conditions in the star's atmosphere, to determine the star's physical characteristics, such as its temperature and the chemical composition of its atmosphere. We input these physical characteristics, and the program calculates a synthetic spectrum, which we compare to the observed spectrum of the star. When the computed spectrum matches all the observed features, we know that the input parameters are the physical parameters of the star.

Ions such as OVI and NeVII, which are atoms missing many electrons, are signs of very high temperatures in the star's atmosphere. The electrons are kicked out from their atoms by high energy photons. Using stellar atmospheres codes, we've determined that the atmosphere of NGC 2371 is extremely hot - about 135,000 degrees Celsius (the Sun is only 5000 degrees!), making it one of the hottest stars known.

How do we ``take the temperature'' of a star?
Spectral features from ions can act as thermometers, giving us precise indication of the star's temperature. In the next figure, we show calculated spectra for stars spanning a wide range of temperatures. In the spectra of the hottest stars, we see a strong OVI feature. But for cooler stars, the signature of OVI gets weaker and we see instead signatures of OV and OIV. What happens is that in these cooler atmospheres, the majority of the oxygen atoms have been stripped of fewer electron. In the coolest stars OVI doesn't appear at all.

When does Neon show up?
Stars with masses similar to the Sun or larger will become CSPN late in their lives. Most CSPN fall into two categories - "hydrogen-rich" (which have abundances of the chemical elements like the Sun) or "hydrogen-deficient" (which have less hydrogen and more carbon, oxygen - and neon). Astrophysicists have developed "stellar evolution codes", which simulate the nuclear reactions going on in the star's core, and how the chemical make-up of the interior of the star changes. Part of the products of the innermost nuclear fusion are brought to the stellar surface, changing the star's appearance. Stellar evolution codes have been able to explain the chemical abundances seen in "hydrogen-rich" CSPN for a while, but the "hydrogen-deficient" stars were still a bit of a puzzle. A stellar evolution scenario recently developed which could explain the high carbon and oxygen abundances seen in these stars, also predicted that the neon abundance would be enhanced. But there was a problem for testing this theory with extremely hot CSPN - no strong neon features (or, "diagnostics") had been identified in their spectra! The neon abundance in a stars' atmosphere is an important clue into which evolutionary path it is on.

You can see the details of our study of this neon feature here.

Cool circumstellar gas in planetary nebulae

CSPN are expected to have both atomic hydrogen ("HI") and molecular hydrogen, or, "H2" (consisting of two hydrogen atoms) in their circumstellar environments. We've found that it is not uncommon for this H2 gas to be much hotter around these stars than that found in the interstellar medium (which typically has temperatures of less than 100 K). If this H2 gas is hot, in can produce a very complicated absorption pattern in the far-UV part of the spectrum. Some illustrative examples are shown below.

Caption: The FUSE data of the DAO white dwarf central star of the Galactic planetary nebula A35 (green). Our TLUSTY model of the star (Teff=80 kK, logg=7.7, z=0.01 Zsun) is shown, both with (blue) and without (red) our HI/H2 absorption models applied. The difference between the red and blue lines is primarily due to H2, and illustrates the complexity of the absorption pattern (Herald & Bianchi 2001, ApJ, 548, 932).

Caption: An expanded view of the FUSE spectrum of A35 is shown (red). We fit the H2 absorption features using a two component model. Our stellar model spectrum with H2 absorption applied from only the "cool" component [logN(H2,cool)=19.6/cm^2,T=200 K] is shown in green, and with both the "cool" and "hot" [N(H2,hot)=17.4/cm^2, T=1250 K] components applied is shown in blue. The cool component alone is not adequate for reproducing all the observed H2 absorption features (Herald & Bianchi 2001, ApJ, 548, 932).

Caption: We found that most CSPN in the Large Magellanic Cloud (LMC) observed with FUSE had hot (T > 2000K) H2 in their circumstellar environments (Herald & Bianchi 2004, ApJ, 611, 294), which severely affects the far-UV spectrum. This figure shows how the absorption pattern of H2 changes and gets more complex as you increase the temperature of the H2 gas. The top plot shows a flat continuum to which are applied the absorption effects of an atomic hydrogen gas with characteristics of the ISM along a sight-line of low reddening [i.e, E(B-V)=0.1, T=80 K, logN(HI)=20.7/cm^2], typical for CSPN in LMC. The middle plot shows the absorption pattern of a typical low-reddening interstellar H2 gas [T=80 K, logN(H2) = 19.7/cm^2], superimposed onto the previous. The bottom plot shows the effects of a relatively small quantity [logN(H2_hot)=16.7/cm^2] of hot (T=5000 K) H2, similar to what we observe toward many of our LMC CSPN, applied onto the previous. The entire FUSE range is affected by the dense field of transitions of numerous ro-vibrational states, which suppress the far-UV continuum. This hot H2, while presenting difficulties to the modeling of the CSPN, gives information on the circumstellar envelope and thus clues to the evolution of these objects (Herald & Bianchi 2003, STScI May Symposium titled "The Local Group as an Astrophysical Laboratory").

Caption: An extreme case of H2 absorption. The FUSE data of the central star of the Galactic planetary nebula NGC 3132 (heavy gray) are shown, along with a black body model with the effects of our HI and H2 absorption models applied (solid), and without (dashed). The H2 gas here was modeled again with two components, with the temperature of the hot component being 1750 K [logN(H2_hot)=17.0/cm^2]. Nebular and interstellar features are labeled below. Transitions of the molecular Hydrogen Werner and Lyman series are marked above (upper and lower rows, respectively). The majority of the features can be attributed to either interstellar or Hydrogen absorption (Herald & Bianchi, in preparation).

Whole list of publications and preprints: click here

R LB-455 Keller, G., Bianchi, L. and Maciel, W. 2014, MNRAS, 442, 1379
UV spectral analysis of very hot H-deficient [WCE] CSPNe: NGC 6905, Pb 6, NGC 5189, NGC 2867 and Sand 3

B LB-432 Keller, G.R. Bianchi, L., , Herald, J. and Maciel, W. 2012, ASPC, 464, 309
Using Grids of High Resolution Synthetic Spectra in the Analysis of [WCE] Stars

R LB-424 Rosenfield, P. Johnson, L.C., Girardi, L., et al. 2012, ApJ, 755, 131
The Panchromatic Hubble Andromeda Treasury. I: Bright UV Stars in the Bulge of M31

I LB-423 Bianchi, L. , 2012, in Proceedings of the International Astronomical Union Symp. 283, eds. A. Manchado, L. Stanghellini, and D. Schonberner, (Cambridge U.P.) p. 45
New Advances in the Field of Planetary Nebulae from Ultraviolet Observations (click here to download pdf)

B LB-422 Keller, G., Bianchi, L. , Herald, J., Maciel, W. 2012, in Proceedings of the International Astronomical Union Symp. 283, eds. A. Manchado, L. Stanghellini, and D. Schonberner, (Cambridge U.P.) p.404
Grids of Synthetic Spectra for H-poor Central Stars of Planetary Nebulae (click here to download pdf)

B LB-421 Bianchi, L. , Manchado, A., and Forster, K., 2012, in "Planetary Nebulae: An Eye to the Future", eds. A. Manchado, L. Stanghellini, and D. Schonberner, Proceedings of the International Astronomical Union Symp. 283, (Cambridge U.P.) p. 308
Ultraviolet Emission Line Imaging of Planetary Nebulae with GALEX (click here to download pdf)

R LB-409 Herald, J., and Bianchi, L. , 2011, MNRAS, 416, 2440
The Winds of Hydrogen-Rich Central Stars of Planetary Nebulae

R LB-407 Keller, G., Herald, J., Bianchi, L. , Maciel, W., and Bohlin, R. 2011, MNRAS, 418, 705
A Grid of Synthetic Models for the Analysis of [WC] CSPNe download pdf

A LB-384 Herald, J., Bianchi, L. , et al. 2010, AAS 215, 606.08
A far-UV Look at Hydrogen-rich CSPN

A LB-382 Keller, G.R., Herald, J., Bianchi, L. , Maciel, W.J. 2010, AAS 215
Synthetic Models for the Analysis of Post-AGB Objects

B LB-362 Herald, J. and Bianchi, L. 2009, in "Future Directions in Ultraviolet Spectroscopy", eds.Van Steenberg et al AIPC, 1135, 151
CSPN wind diagnostic lines in the Far-UV download ps file

B LB-348 Bianchi, L., Seibert, M., Herald, J., Thilker, D. 2008, in "Space Astronomy: The UV Window to the Universe", eds. A.I. Gomez de Castro, el. publ.
Ultraviolet Imaging of Planetary Nebulae

B LB-343 Bianchi, L., and Herald, J.E. 2007, in ``UV Astronomy: Stars from Birth to Death'', eds. A.I. Gomez de Castro and M. Barstow, UCM Editorial Complutense, p. 89 (proceedings of JD4 during the IAU GA 2006) [ps file]
New powerful diagnostics for hot evolved stars: constraining the hottest temperatures, the faintest winds, and the neon abundance.

R LB-321 Herald, J.E. and Bianchi, L., 2007, ApJ, 661, 845
Central Stars of Planetary Nebulae in the Magellanic Clouds: a Detailed Spectroscopic Analysis

B LB-318 Bianchi, L., and Herald, J.E. 2007, in ``The UV Universe: Stars from Birth to Death'', eds. A.I. Gomez de Castro and M. Barstow, in press (proceedings IAU GA 2006 JD04) [ps file]
New powerful diagnostics for hot evolved stars: constraining the hottest temperatures, the faintest winds, and the neon abundance.

A LB-281 Herald, J., Bianchi, L. , 2005, AAS 207, 182.24
Neon VII: A Powerful Wind Diagnostic For Very Hot Stars

R LB-270 Herald, J., Bianchi, L. and Hillier, D.J., 2005, ApJ, 627, 424 [link]
Discovery of NeVII in the winds of hot evolved stars

R LB-194 Herald, J. and Bianchi, L. 2004, ApJ, 609, 378
Far-UV spectroscopic analysis of four central stars of Planetary nebulae

R LB-193 Herald, J. and Bianchi, L. 2004, PASP, 116, 391
A far-UV spectroscopic analysis of the Central Star of the Planetary Nebula Longmore 1
click to download the PDF file - - - click to download the PS file - - - link to PASP

R LB-192 Herald, J. and Bianchi, L. 2004, ApJ, 611, 294
Central Stars of Planetary Nebulae in the Large Magellanic Cloud: A Far-UV Spectroscopic Analysis
click here to download the pdf file -- OR -- click here for the ApJ link

LB-174 Herald, J. and Bianchi, L., 2003, in "Winds, Bubbles and Explosions", RMxAA, 15, 73
Properties of Galactic and LMC CSPN disclosed by FUSE observations

LB-172 Herald, J., and Bianchi, L., 2002, ApJ, 580, 434
The binary central star of the Planetary Nebula Abell 35

Herald, J., and Bianchi, L., 2002, AAS199, 13.04
FUSE studies of Galactic and LMC CSPN
(click here to view the entire poster)

Bianchi et al. 2001 , AJ, 122, 1538
HST and Ground-Based Spectroscopy of K648 in M15

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