by neufer » Wed Oct 27, 2010 8:39 pm
Ann wrote: I believe that the red star in this picture is the same as the blue star in the GALEX image. But how can a star that is very red visually be very blue in ultraviolet light? Is that even possible?
Apparently so. The star in question would be EG Andromedae, a cool red M giant. EG Andromedae is very far away, possibly as much as 2,000 light-years, and it may be around 500 times as bright as the Sun in visual light. That, of course, means that it would be even brighter in infrared light, given the fact that this star is so cool. Its color index is 1.613 ± 0.014, which is a lot.
So EG Andromeda is a visually very red and cool star. But it is also a massive star, and its core is blazingly hot. Even though EG Andromedae is visually a red star, its core emits a lot of far ultraviolet light, and the star will show up as strikingly blue in a GALEX image.
Its core is blazingly hot? You can see the core??
- http://www.youtube.com/watch?v=1Dk83DYRgp0
There once was an M giant in Andromeda
Who watched little boys as they played with their
Marbles and toys as in days of old yore
And for a companion he had a white dwarf.
EG Andromeda is a cool red M giant with a very hot companion:
http://iopscience.iop.org/1538-3881/128/4/1790/204122.text.html wrote:
SYNTHETIC SPECTRAL ANALYSIS OF THE HOT COMPONENT IN
THE S-TYPE SYMBIOTIC VARIABLE EG ANDROMEDAE
KELLY KOLB, JOLEEN MILLER, AND EDWARD M. SION
Department of Astronomy and Astrophysics, Villanova University.
AND JOANNA MIKOłAJEWSKA
Centrum Astronomiczne im. Mikołaja Kopernika, Polska Akademia Nauk, ulica Bartycka 18, PL-00-716 Warszawa, Poland.
Received 2004 March 29; accepted 2004 June 25
ABSTRACT
We have applied grids of non-LTE (NLTE) high-gravity model atmospheres and optically thick accretion disk models for the first time to archival IUE and FUSE spectra of the S-type symbiotic variable EG Andromedae taken at superior spectroscopic conjunction, when Rayleigh scattering should be minimal and the hot component is viewed in front of the red giant. For EG And's widely accepted, published hot-component mass, orbital inclination, and distance from the Hipparcos parallax, we find that hot, high-gravity, NLTE photosphere model fits to the IUE spectra yield distances from the best-fitting models that agree with the Hipparcos parallax distance, but at temperatures substantially lower than the modified Zanstra temperatures. NLTE fits to an archival FUSE spectrum taken at the same orbital phase as the IUE spectra yield the same temperature as the IUE temperature (50,000 K). However, for the same hot-component mass, inclination, and parallax-derived distance, accretion disk models at moderately high inclinations, ∼60°–75°, with accretion rates from 1 × 10-8 to 1 × 10-9 M⊙ yr-1 for white dwarf masses Mwd = 0.4 M⊙ yield distances grossly smaller than the distance from the Hipparcos parallax.
Therefore, we rule out an accretion disk as the dominant source of the far-UV flux. Our findings support a hot bare white dwarf as the dominant source of far-UV flux.
[quote="Ann"] I believe that the red star in this picture is the same as the blue star in the GALEX image. But how can a star that is very red visually be very blue in ultraviolet light? Is that even possible?
Apparently so. The star in question would be EG Andromedae, a cool red M giant. EG Andromedae is very far away, possibly as much as 2,000 light-years, and it may be around 500 times as bright as the Sun in visual light. That, of course, means that it would be even brighter in infrared light, given the fact that this star is so cool. Its color index is 1.613 ± 0.014, which is a lot.
So EG Andromeda is a visually very red and cool star. But it is also a massive star, and its core is blazingly hot. Even though EG Andromedae is visually a red star, its core emits a lot of far ultraviolet light, and the star will show up as strikingly blue in a GALEX image.[/quote]
[b][color=#FF0000][size=150]Its core is blazingly hot? You can see the core??[/size][/color][/b]
[list][b] http://www.youtube.com/watch?v=1Dk83DYRgp0[/b]
There once was an M giant in Andromeda
Who watched little boys as they played with their
Marbles and toys as in days of old yore
And for a companion he had a white dwarf.[/list]
EG Andromeda is a cool red M giant with a very hot companion:
[quote=" http://iopscience.iop.org/1538-3881/128/4/1790/204122.text.html"]
SYNTHETIC SPECTRAL ANALYSIS OF THE HOT COMPONENT IN [b][color=#FF00FF]THE S-TYPE SYMBIOTIC VARIABLE EG ANDROMEDAE[/color][/b]
KELLY KOLB, JOLEEN MILLER, AND EDWARD M. SION
Department of Astronomy and Astrophysics, Villanova University.
AND JOANNA MIKOłAJEWSKA
Centrum Astronomiczne im. Mikołaja Kopernika, Polska Akademia Nauk, ulica Bartycka 18, PL-00-716 Warszawa, Poland.
Received 2004 March 29; accepted 2004 June 25
ABSTRACT
We have applied grids of non-LTE (NLTE) high-gravity model atmospheres and optically thick accretion disk models for the first time to archival IUE and FUSE spectra of the S-type symbiotic variable EG Andromedae taken at superior spectroscopic conjunction, when Rayleigh scattering should be minimal and the hot component is viewed in front of the red giant. For EG And's widely accepted, published hot-component mass, orbital inclination, and distance from the Hipparcos parallax, we find that hot, high-gravity, NLTE photosphere model fits to the IUE spectra yield distances from the best-fitting models that agree with the Hipparcos parallax distance, but at temperatures substantially lower than the modified Zanstra temperatures. NLTE fits to an archival FUSE spectrum taken at the same orbital phase as the IUE spectra yield the same temperature as the IUE temperature (50,000 K). However, for the same hot-component mass, inclination, and parallax-derived distance, accretion disk models at moderately high inclinations, ∼60°–75°, with accretion rates from 1 × 10-8 to 1 × 10-9 M⊙ yr-1 for white dwarf masses Mwd = 0.4 M⊙ yield distances grossly smaller than the distance from the Hipparcos parallax. [b][color=#FF00FF]Therefore, we rule out an accretion disk as the dominant source of the far-UV flux. Our findings support a hot bare white dwarf as the dominant source of far-UV flux[/color][/b].[/quote]