Note that Biretta & Kozhurina-Platais (2004) have confirmed empirically that the ACS position-angle design specifications were met closely (within 03) in the flight instrument. Figure 8. 499 75 501 0 obj<>stream

Figure 11.

In addition to the polarimetric images, we make use below of B- and I-band direct images that were obtained contemporaneously with the polarimetric observations of 2002 December 17. Second, the presence of a substantial number of stars in the FOV complicates the analysis. 0000017038 00000 n RIS. The observer lies in the direction of the +z axis. V838 Mon itself is located near the center of the cavity in the 2002 May 20 image. The bandpass filter chosen for our first two observations was the ACS "B" filter (F435W), and for the remaining four observations it was the "Broad V" filter (F606W). Figure 7 shows a color rendition of the 2002 December 17 observations prepared by one of us (Z.L.) Many of the stars are significantly saturated, so our photometric procedure used a correction for saturation based on the ratio of flux in an unsaturated annulus to the total flux. The apparent magnitude of V838 Mon at its maximum in early 2002 February was V = 6.77 (see Figure 5). Also shown are the data (left-hand noisy line) and a similar fit (left-hand smooth curve) for the 2002 September polarimetric data, which yield a best-fit distance of 5.6 kpc.

These curves were calculated from the fully cleaned (7 × 7)-binned polarization images, derived as described in Sections 3.3–3.5, and illustrated in Figure 4. These issues do not affect our distance determination, and are thus beyond the scope of the present paper; see the recent conference proceedings for discussions of the origin of the dust (Corradi & Munari, eds., 2007).
0000024692 00000 n The directions of the electric vectors are indicated by the red lines, whose lengths are proportional to the degree of polarization. Quick Facts: V838 Monocerotis.

0000028753 00000 n First, to obtain a sense of the approximate distance implied by these data, we located the peaks of the polarization rings for 2002 September and December. Figure 5 shows the light curves that we adopted. For each numerical model, at each assumed distance, we calculated the intensity-weighted mean scattering angle as a function of radius, taking into account the geometry, light curve, and scattering functions described above. We then drizzled the error images to register them spatially with the drizzled data images, adjusting them to units of count rates per second.

We fitted parabolas to the peaks of the phase functions, using only polarization values p>0.3, in order to derive the locations of the peaks as a function of the assumed distance. In Figure 3 we represent the linear polarization in the image of 2002 December 17, using red line segments, which are oriented to show the direction of the electric vectors and whose lengths are proportional to the degree of polarization. Outburst light curves adopted for V838 Mon in the ground-based Johnson–Kron–Cousins B, V, and I bandpasses, colored blue, green, and red respectively. S = 15 Mon falls among the stars of the young cluster NGC 2264, is surrounded by a bright nebula called Sharpless 273, which it helps excite, and lies to the north of one of the fantastic sights of the sky, the “Cone Nebula,” a long dark interstellar dust cloud that is being evaporated at its tip by another nearby hot star. With this definition, summation of the voxels along each LOS yielded an ACS count rate for each spatial pixel for an assumed constant density of nH = 1 cm−3. 0000014545 00000 n Setting the radius of this ring to a linear size of ct yielded initial distance estimates of 5.4–6.2 kpc.

0000013162 00000 n The 2002 May image shows that the linear polarization increases inward toward the star, but does not reach a maximum before encountering the prominent cavity surrounding the star. As the above equation shows, the 90° scattering, occurring for dust at z = 0, takes place at a linear distance of x = ct from the star. 0000025947 00000 n (2004) and in the ACS Instrument Handbook (Pavlovsky et al. Thus, if the peak polarization were to occur 5° on either side of 90°, the derived distance would change by about 9%, corresponding to a distance range of 5.5–6.7 kpc.

This expectation appears to be borne out by the recent discovery of an apparently V838 Mon-like event in the Virgo galaxy M85 (Rau et al. 0000027356 00000 n Please note, The Astrophysical Journal Letters (ApJL) and Research Notes of the AAS (RNAAS)


The data from 2005 December were also processed as described above. As the Columbia University astrophysicist Caleb Scharf pointed out in an essay on his blog recently, long after you and I are dead, the light reflected off our faces today will still be traveling across space, ever fainter with distance, but always there. 0000003346 00000 n v838 monocerotis: a geometric distance from hubble space telescope polarimetric imaging of its light echo * William B. Sparks 1 , Howard E. Bond 1 , Misty Cracraft 1 , Zolt Levay 1 , Lisa A. Crause 2 , Michael A. Dopita 3 , Arne A. Henden 4 , Ulisse Munari 5 , Nino Panagia 1,6,7 , Sumner G. Starrfield 8 , Ben E. Sugerman 9 , R. Mark Wagner 10 , and Richard L. White 1

The result agrees extremely well with the distance obtained from a completely independent, classical method of cluster main-sequence fitting, thus providing strong support for the validity of the polarimetric technique. a Position angle of the electric vector measured counter-clockwise from the spacecraft V3 axis. As a prelude to the polarization analysis, it is useful to visualize the geometry of the light echo at each of the epochs of observation. The use of light-echo polarimetry to obtain geometric distances of supernovae (SNe) was proposed by Sparks (1994). Using the proper paraboloidal geometry, Bond et al. The American Astronomical Society. See the text for details. (2003), but are plotted here as stellar magnitudes rather than on a linear flux scale. By continuing to use this site you agree to our use of cookies. Our results strongly verify the polarimetric method for determining geometric distances. Others have proposed that V 838 was actually two stars orbiting each other, and that the explosions were a result of their atmospheres merging into a common envelope of gas.

The American Astronomical Society (AAS), established in 1899 and based in Washington, DC, is the major organization of professional astronomers in North America.

In 2005 December the echo was much fainter, and we used 11 × 11-pixel smoothing.

Following a careful reduction of polarimetric images of the light echo, obtained with the ACS onboard the HST, we confirmed the presence of an apparently expanding ring of highly polarized light surrounding V838 Mon. where nH(z) is the volume density of hydrogen atoms, ω is the dust albedo, and σ is the scattering cross section per atom. The light echo is about 50% linearly polarized at the location of the maximum. V838 Monocerotis (Nova Monocerotis 2002) is a cataclysmic variable - or outburst - star in the constellation Monoceros about 20,000 light years (6 kpc) from the Sun.

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