* Astronomy

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RE: V838 Monocerotis

Blobrana · L

For a brief moment in 2002, an obscure star called V838 Monocerotis (nicknamed V838 Mon by astronomers) suddenly became 600,000 times brighter than our Sun and temporarily was the brightest star in our Milky Way galaxy. Within a few months, it faded back into obscurity.

While the star has fascinated astronomers worldwide ever since, the source of the star's sudden outburst remains a mystery. Determined scientists remain hopeful they will learn more about the nature this stellar eruption by pointing a variety of telescopes at V838 Mon and its surrounding environment.
In one observation, NASA's Spitzer Space Telescope discovered an infrared light echo around V838 Mon. This is only the second infrared light echo ever to be resolved, and its detection has helped astronomers gain some valuable insights into the star's "personality."

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Title: Hubble Space Telescope Observations of the Light Echo around V838 Monocerotis
Authors: Howard E. Bond (Space Telescope Science Institute)

The outburst of V838 Monocerotis in early 2002, and the subsequent appearance of its light echoes, occurred just before the installation of the Advanced Camera for Surveys into the Hubble Space Telescope (HST). This fortunate sequence of events has allowed us to obtain spectacular HST images of the echoes, yielding not only pictures of extraordinary beauty, but also providing unique scientific information. Our team has used the HST images to provide a direct geometrical distance to V838 Mon, based on polarimetric imaging, and limits on the distance based on the apparent angular expansion rates. Several morphological features seen in the HST images strongly suggest that the illuminated dust was ejected from the star in a previous outburst, similar to the current one. In particular, a "double-helix" feature points exactly back to the star. Moreover, three-dimensional mapping of the outer edges of the dust suggests an overall ellipsoidal shape, centred on V838 Mon itself. And the appearance of the light echo in the most recent Hubble images is remarkably similar to that of a well-known planetary nebula, M27. Future work on the HST images will include an analysis of interstellar-dust physics, in a situation where the scattering angle and illumination are unambiguously known, and visualisation of a fully three-dimensional map of the dust distribution.

HST image of V838 Mon on 2005 November 17. The surface brightness is highest along an "equatorial plane" passing through the star, marked with a dashed line to guide the eye.

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Title: A Young Open Cluster Surrounding V838 Monocerotis
Authors: Howard E. Bond, Melike Afsar (Space Telescope Science Institute; Department of Astronomy and Space Sciences, Ege University)

During a program of spectroscopic monitoring of V838 Mon, researchers serendipitously discovered that a neighbouring 16th-mag star is of type B. They then carried out a spectroscopic survey of other stars in the vicinity, revealing two more B-type stars, all within 45" of V838 Mon. The researchers have determined the distance to this sparse, young cluster, based on spectral classification and photometric main-sequence fitting of the three B stars. The distance is found to be 6.2 ±1.2 kpc, in excellent agreement with the geometric distance to V838 Mon of 5.9 kpc obtained from Hubble Space Telescope polarimetry of the light echoes. The cluster's age is less than 25 Myr.
The absolute luminosity of V838 Mon during its outburst, based on their distance measurement, was very similar to that of M31 RV, an object in the bulge of M31 that was also a cool supergiant throughout its eruption in 1988. However, there is no young population at the site of M31 RV.
It does not appear possible to form a nova-like cataclysmic binary system within the young age of the V838 Mon cluster, and the lack of a young population surrounding M31 RV suggests that the outburst mechanism does not require a massive progenitor. These considerations appear to leave stellar-collision or merger scenarios as one of the remaining viable explanations for the outbursts of V838 Mon and M31 RV.

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Hubble image of V838 Mon and its light echo. The three neighbouring B-type stars discovered in their work are circled, along with V838 Mon itself. From top to bottom, the circled stars are V838 Mon and stars 7, 9, and 8. Other bright stars in the image have proven to be foreground stars.

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-- Edited by Blobrana at 03:56, 2006-08-11

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Title: V838 Mon and M31-RV: The Stellar Populations Angle
Authors: Michael H. Siegel, Howard E. Bond

Insight into the origin of unusual events like the eruption of V838 Mon can be obtained from studies of the stellar populations from which they arise. V838 Mon lies in an intriguing region of the Galaxy, toward the warped outer edge of the disk, with significant contributions from the Galactic thick disk and the recently discovered Monoceros tidal stream. The initial distance measures placed V838 Mon in a jumbled region of the Galaxy but the recent shorter distances make it highly likely that V838 Mon was a thin disk star -- likely in a spiral arm -- consistent with the recent detection of a young cluster in the vicinity. We compare V838 Mon to M31-RV, a red variable that erupted in the bulge of M31 in 1988 and had a peak luminosity and spectral evolution very similar to V838 Mon. Archival HST images show no nebulosity or unusual stars at M31-RV's projected location. Moreover, the only stellar population in the field is a canonic old bulge population. This indicates that whatever the origin of the red novae, the mechanism is likely independent of age and progenitor mass. In particular, the B3V star seen in V838 Mon it not a necessary part of the eruption mechanism.

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Title: Observations of V838 Mon and the nearby region in the CO J = 1-0, 2-1 and 3-2 transitions
Authors: T. Kaminski, M. Miller, R. Szczerba, R. Tylenda

Researchers present observations of V838 Mon and its close vicinity in the three lowest rotational transitions of CO. The J = 2-1 and 3-2 data were obtained using the 3 m KOSMA telescope. They include on-the-fly maps covering a large area (~3.4 sq. deg.) around V838 Mon and long integrations on the star position. Complementary observations in the CO J = 1-0 transition were obtained using the 13.7 m Delingha telescope. The star position as well as 25 other points preselected in the near vicinity of the object have been measured in this transition.
They report on a detection of two narrow emission components in J = 2-1 and 3-2 transitions at the position of V838 Mon. Lines were found at radial velocities of V_lsr=53.3 km/s and V_lsr=-11.0 km/s. Their origin is unclear. They also discuss results of the observations of the vicinity of V838 Mon.

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Title: Interferometric Observations of Explosive Variables: V838 Mon, Nova Aql 2005, and RS Oph
Authors: Benjamin F. Lane (MIT), Alon Retter (Penn State), Joshua A. Eisner (UC Berkeley), Robert R. Thompson (MSC, Caltech), Matthew W. Muterspaugh (Caltech)

During the last two years researchers have used the Palomar Testbed Interferometer to observe several explosive variable stars, including V838 Monocerotis, V1663 Aquilae and recently RS Ophiuchi.
They observed V838 Monocerotis approximately 34 months after its eruption, and were able to resolve the ejecta. Observations of V1663 Aql were obtained starting 9 days after peak brightness and continued for 10 days. The researchers were able to resolve the milliarcsecond-scale emission and follow the expansion of the nova photosphere. When combined with radial-velocity information, these observations can be used to infer the distance to the nova.
Finally they have resolved the recurrent nova RS Oph and can draw some preliminary conclusions regarding the emission morphology.

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Title: Variability of V838 Mon Before Its Outburst
Authors: S. Kimeswenger (Astro- u. Teilchenphysik Innsbruck)

Archival observations of the progenitor of V838 Mon were re-calibrated as well as multiple images from the 2MASS and the DENIS near infrared surveys. Both - the better calibrated values as well as the variability found in the photographic material has strong effects for the fitting of the spectral energy distribution of the progenitor.

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Multi-wavelength V838 Mon images.
Column 1: MIPS 24, 70, and 160 µm images (top to bottom); Column 2: PSF subtracted images (no point source detected at 160 µm); Column 3: HST F814W image convolved to the MIPS resolution at 24, 70, and 160 µm images (top to bottom) after removal of field stars in the HST image. MIPS beam sizes are indicated by the white circles with FWHM of 6'', 18'', and 40'' .
All images are displayed in a field of view of 160'' by 160'', with N up and E toward the left, and in false-colour logarithmic scale with brightness and contrast adjusted for best presentation of each image.
The displayed surface brightness (top to bottom) ranges from 10-² to 10-² mJy/arcsec² at 24 µm, 5×10^-1 to 8 mJy/arcsec² at 70 µm, 6×10^-1 to 2 mJy/arcsec² at 160 µm for the first column; and from 3×10² to 5×10-1 mJy/arcsec² at 24 µm, and 3×10^-1 to 4 mJy/arcsec² at 70 µm for the second column.