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September 18, 2007 Hydrogen-Deficient Stars, Tuebingen
Transient Mass-Loss Events in the PG 1159 [WCE] Central Star of Longmore 4
Howard E. BondSpace Telescope Science Institute
Baltimore, USA
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Longmore 4
• Faint PN discovered by Longmore (1977), during ESO-SRC sky survey.
• Central star (V=16.6) classified as PG 1159 star by Méndez et al. (1985):– Conspicuous C IV and He II absorption, no
Balmer absorption. O VI 3811-3834 in emission– H-def spectral type: O VI , O(C), or PG 1159– Spectrum similar to K 1-16, the first known
pulsating PNN
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Pulsations of Lo 4
• CCD photometry of Lo 4 nucleus (Bond & Meakes 1990) revealed GW Vir-like non-radial pulsations.– Second known pulsating PNN– Strong periodicity near 31 min (1850 sec); at
least 9 pulsation modes, with periods of 831 to 2325 sec
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Pulsations of Lo 4
Bond & Meakes 1990
31 min
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Lo 4 and Other GW Vir
Hydrogen-Deficient Pulsating
PNNi and WDs
Ciardullo & Bond 1996
Lo 4
NGC 246
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A Mass-Loss Event in Lo 4• A spectacular transient mass-loss event in
Lo 4 was discovered by Werner et al. (1992,1993)– Spectrum changed from PG 1159 to [WCE]
([WC2]-[WC3]) and then back to PG 1159
– Teff = 170,000 K, log g = 6 (Werner et al. 2007)
– During [WC] state: log (dM/dt) = 7.3 (M/yr), v = 4000 km/s
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The 1992 EventWerner et al. (1992)
C IV+He II C IVO VI
He II
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The Werner et al. Mass-Loss Event
• Spectrum on 1991 May 19: normal PG 1159
• 1992 Jan 27 & 28: [WCE]! Emission weaker on 2nd night
• 1992 Jan 31: [WC] emiss. present but weak
• 1992 Feb 13: normal PG 1159
• A unique phenomenon, never seen before in hot post-AGB stars
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Spectroscopic Monitoring of Lo 4• I began spectroscopic monitoring in 2003—
• —using service observing with SMARTS Consortium 1.5m telescope at Cerro Tololo– Wavelength coverage 3650-5400 Å– Resolution 4.3 Å– S/N per res. element ~30-40 on good nights
• 2003 Mar to 2007 Jul: 35 usable spectra
• Two mass-loss events were detected!
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SMARTS 1.5m Spectra
2003
2007
mean sp.
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SMARTS Spectra, Mean Subtracted
2003
2007
2006 Jan 16
2006 Nov 30
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The 2006 Outbursts
2006 Jan 16
2006 Nov 30
mean normal spectrum
(spectrum 53 days earlier was normal)
(spectrum 56 days earlier was normal)
C IV+He II
O VI
O VI
C IV He II
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Outburst Decay Timescale
2006 Jan 16
2006 Nov 30
15 days later
17 days later
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Central Star of NGC 246• PG 1159 spectrum similar to Lo 4
– Teff = 150,000 K, log g = 5.7 (Rauch & Werner 1997) (slightly cooler than Lo 4)
• Shows GW Vir-type pulsations similar to Lo 4 (Ciardullo & Bond 1996)– periods of 24-31 min– lower light amplitudes than Lo 4
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Central Star of NGC 246• From 2003 to 2007 ~100 spectra obtained
with SMARTS 1.5m
• No mass-loss events were detected
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Constraints on Mechanism• The transient wind is hydrogen-deficient
– Therefore it does not seem to be related to an accretion event involving a companion star, debris disk, infalling planets, or other exotic external cause
• Short durations of events mass loss not driven on evolutionary timescale
• The similar, but lower-amplitude, pulsator NGC 246 has not shown such events
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Speculations• Córsico et al. (2006) suggest Lo 4 is near
red edge of GW Vir instability strip & this may be related to transient events– But new Teff = 170,000 K (Werner et al. 2007)
moves it further away from red edge
• Lo 4 does lie near the boundary between [WCE] and PG 1159 PNNi—see next slide
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Locations in HR Diagram
Figure from Werner et al. 2004
Lo 4 NGC 246
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Speculations, continued• Since Lo 4 lies near boundary between
[WCE] and PG 1159 PNNi, it may take only a small perturbation to cause transition to [WCE]
• For example, occasionally many pulsation modes will be in phase, giving a temporarily large amplitude—could this trigger the outbursts?– If so, outbursts will recur periodically
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Time-Averaged Mass-Loss Rate
• 3 mass-loss events seen in ~40 randomly timed spectra
Lo 4 is in “high” state ~8% of time
• Time-averaged dM/dt due to outbursts is ~ 0.08 107.3 ~ 4 109 M/yr
• dM/dt during normal PG 1159 state is ~3-30 109 (from FUSE data; Werner priv. comm.) outbursts have little additional effect on evolution
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No Ejecta Visible Near Lo 4 in Shallow HST Image
HST/WFPC2 F555W 140 sec Width 9″1995 GO-6119 (Bond et al.)
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Conclusions• Lo 4 occasionally transitions from its
normal PG 1159 spectrum to [WCE]
• This supports the commonly accepted evolutionary link between the two types
• Further observations could test whether the outbursts occur periodically (beat period)
• Other PNNi near the [WCE] - PG 1159 transition should be monitored.
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The Hubble Heritage Gallery
Visit our website: http://heritage.stsci.edu
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The Hubble Heritage Program
• Founded in 1998 to bring the most compelling Hubble images to the public
• Main criterion is pictorial beauty, with scientific interest also considered
• Images are taken from archive, sometimes supplemented by new observations obtained by the Heritage team through Director’s Discretionary time, and processed for release
And now for a commercial announcement…
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The Hubble Heritage Program
• Some observations are entirely new images obtained by Heritage team (e.g., M82 Galaxy image for Hubble 16th anniversary)
• Prizes & honors:– Images on US & British postage stamps– 2003 Klumpke-Roberts Award of the
Astronomical Society of the Pacific for contributions to public appreciation of astronomy
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More Conclusions• Other PNNi show spectral variations