arXiv subjects

R. P. Stefanik

Publications and source records attributed to R. P. Stefanik.

41 records · Page 3Linked to original sources

arXiv2003

Spectroscopy of the Millennium Outburst and Recent Variability of the Yellow Hypergiant Rho Cassiopeiae

In the summer and fall of 2000 the yellow hypergiant Rho Cassiopeiae dimmed by more than a visual magnitude, while its effective temperature decreased from \~7000 K to below 4000 K over ~200 d. We observed the highest mass-loss rate of ~5 % of the solar mass per year in a single stellar eruption so far (Lobel et al. 2003, ApJ, 583, 923). We determine from newly formed TiO bands a gas mass-loss rate of M_dot ~ 5.4 10^{-2} Msun/yr, which is of the same order of magnitude as has been proposed for the giant outbursts of the Luminous Blue Variable Eta Carinae. Over the past two years since the outburst event we observe a very prominent inverse P Cygni profile in Balmer H alpha. Strong Halpha emission has not before been observed in the hypergiant over this long period of time, signaling an unusual strong collapse of the upper hydrogen atmosphere, which we also observed in the months before the 2000 outburst. Since the brightness decrease of March 2003 we observe a transformation of the H alpha profile into a P Cygni profile, signaling a strong expansion of Rho Cas's upper atmosphere, which could possibly result in a new outburst event. High-resolution spectroscopic observations reveal that the lower photosphere rapidly expanded until May-June 2003. Very recent high-resolution observations of September 2003 show however that the photospheric absorption lines did not shift far blueward as observed in July 2000 before the strong brightness decrease of the outburst. The Fe I 5572 A is redshifted, signaling the collapse of the lower photosphere. A new strong brightness decrease by more than a magnitude in V for the fall and winter of 2003 is therefore not expected.

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arXiv2003

Spatially Resolved STIS Spectroscopy of Betelgeuse's Outer Atmosphere

We present spatially resolved spectra observed with HST-STIS of the upper chromosphere and dust envelope of Alpha Orionis (M2 Iab). In the fall of 2002 a set of five high-resolution near-UV spectra was obtained by scanning at intensity peak-up position and four off-limb target positions up to one arcsecond, using a small aperture, to investigate the thermal conditions and flow dynamics in the outer atmosphere of this important nearby cool supergiant star. Based on Mg II h & k, Fe II 2716 A, C II 2327 A, and Al II ] 2669 A emission lines we provide the first evidence for the presence of warm chromospheric plasma at least 1 arcsecond away from the star at ~40 R* (1 R*~700 Rsun). The STIS spectra reveal that Betelgeuse's upper chromosphere extends far beyond the circumstellar H alpha envelope of ~5 R*, determined from previous ground-based imaging. The flux in the broad and self-absorbed resonance lines of Mg II decreases by a factor of ~700 compared to the flux at chromospheric disk center. We observe strong asymmetry changes in the Mg II h and Si I resonance line profiles when scanning off-limb, signaling the outward acceleration of gas outflow in the upper chromosphere. From the radial intensity distributions of Fe I and Fe II emission lines we determine the radial non-LTE iron ionization balance. We compute that the local kinetic gas temperatures of the warm chromospheric gas component in the outer atmosphere exceed 2600 K, when assuming local gas densities of the cool gas component we determine from radiative transfer models that fit the 9.7 um silicate dust emission feature. The spatially resolved STIS spectra directly demonstrate that warm chromospheric plasma co-exisists with cool gas in Betelgeuse's circumstellar dust envelope.

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arXiv2003

When Do Planets Form? A Search for Extra-solar Planets Around Metal-Poor Stars

We present preliminary results from our spectroscopic search for planets within 1 AU of metal-poor field dwarfs using NASA time with HIRES on Keck I. The core accretion model of gas giant planet formation is sensitive to the metallicity of the raw material, while the disk instability model is not. By observing metal-poor stars in the field we eliminate the role of dynamical interactions in dense stellar environments, such as a globular cluster. The results of our survey should allow us to distinguish the relative roles of the two competing giant planet formation scenarios.

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arXiv2003

High-Resolution Spectroscopy of the Yellow Hypergiant Rho Cassiopeiae from 1993 Through the Outburst of 2000-2001

We present an overview of the spectral variability of the peculiar F-type hypergiant Rho Cas, obtained from our long-term monitoring campaigns over the past 8.5 years with four spectrographs in the northern hemisphere. Between 2000 June and September an exceptional variability phase occurred when the V-brightness dimmed by about a full magnitude. The star recovered from this deep minimum by 2001 April. It is the third outburst of Rho Cas on record in the last century. We observe TiO absorption bands in high-resolution near-IR spectra obtained with the Utrecht Echelle Spectrograph during the summer of 2000. TiO formation in the outer atmosphere occurred before the deep brightness minimum. Atmospheric models reveal that the effective temperature decreases by at least 3000 K, and the TiO shell is driven supersonically with Mdot ~= 5.4 10^{-2} Msun/yr. Strong episodic mass loss and TiO have also been observed during the outbursts of 1945-47 and 1985-86. A detailed analysis of the exceptional outburst spectra is provided, by comparing with high-resolution optical spectra of the early M-type supergiants Mu Cep (Ia) and Betelgeuse (Iab). The outburst spectra indicate the formation of a low-temperature, optically thick circumstellar gas shell of 3 10^{-2} Msun during ~200 d, caused by dynamic instability of the upper atmosphere of this pulsating massive supergiant near the Eddington luminosity limit. We observe that the mass-loss rate during the outburst is of the same order of magnitude as has been proposed for the outbursts of Eta Carinae. We present calculations that correctly predict the outburst time-scale, whereby the shell ejection is driven by the release of hydrogen ionization-recombination energy.

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arXiv2002

The Millennium Outburst of the Cool Hypergiant Rho Cassiopeiae: Spectroscopy and Modeling

Between 2000 June and September an exceptional variability phase occurred in the peculiar F-type hypergiant Rho Cas, when the V-brightness dimmed by at least a full magnitude. The star recovered from this deep minimum by 2001 April. It is the third outburst of Rho Cas on record in the last century. We observe TiO absorption bands in high-resolution optical and near-IR spectra obtained with the Utrecht Echelle Spectrograph during the summer of 2000. TiO formation in the outer atmosphere occurred before the deep brightness minimum.Atmospheric models reveal that the effective temperature decreases by at least 3000 K, and the TiO shell is driven supersonically with Mdot =~5.4 10^-2 Msun/yr. Strong episodic mass loss and TiO have also been observed during the outbursts of 1945-47 and 1985-86. An analysis of the exceptional outburst spectra of 2000-01 is provided, by comparing with high-resolution optical spectra of the early M-type supergiants Mu Cep (Ia) and Betelgeuse (Iab). During the outburst, central emission appears above the local continuum level in the split Na D lines. The outburst spectra indicate the formation of a low- temperature, optically thick circumstellar gas shell of 3 10^-2 Msun during \~200 d, caused by dynamic instability of the upper atmosphere of this pulsating massive supergiant near the Eddington luminosity limit. We present an equation that correctly predicts the outburst time-scale, whereby the shell ejection is driven by the release of hydrogen recombination energy. We observe that the mass-loss rate during the outburst is of the same order of magnitude as has been proposed for the outbursts of Eta Carinae. The research results in this paper are described in further detail in Lobel et al. (2003). A spectral movie sequence of the outburst is available at http://cfa-www.harvard.edu/~alobel/

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