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V475 Sct [aka Nova Scutum 2003]

 

Nova V475 Scutum (Sct) was discovered on August, 28.58, 2003 UT by Hideo Nishimura of Kakegawa, Shizuoka-ken, Japan.  The position of the nova was reported as RA 18h 49m 38s   Dec -9o 33' 45" equinox 2000.0.  The discovery magnitude was reported as 8.5.  V475 Sct is unique in that the original discovery by Nishimura was detected on photographic film using a 200 mm telephoto lens. 

The area of sky in which nova V475 Sct occurred was well documented before the occurrence.  See Image 1.  The star from which the nova forms is called the progenitor.  The progenitor for V475 Sct is documented in the Guide Star Catalog II [GSC2 2001] as GSC2 S300220349902 with a magnitude of B=17.2.

The Physics of a Nova 

The White Dwarf and Companion Star

It is widely accepted that novae are close binary stars. The central star is a white dwarf and the companion star is a late-type main-sequence star that still contains hydrogen in its outer shell.  The white dwarf begins its life as a star of less than four solar masses  that has fused its supply of hydrogen into helium, carbon, oxygen neon and magnesium.  The white dwarf then becomes a low volume star which has consumed all of its hydrogen, ejecting much of its mass into the formation of a local nebula cloud.  The mass of the white dwarf is sufficient that the electrons in the core become degenerate, or disengaged from their original atoms.  The now ionized elements of carbon, oxygen, neon and magnesium align themselves in a crystalline-like lattice that allows for the maximum density of the star's mass. 

At this point, the white dwarf's mass is now greater than 1.4 solar masses

When a sufficient amount of hydrogen has accumulated, the gravitational pressure on the hydrogen is so great that a thermonuclear fusion reaction occurs at the star's surface.  This explosion causes either a wind or a shell that blows some of the hydrogen and small amounts of the star's core out into space.  It is this formation of either a wind or a shell that determines the type of nova that has been produced.  The wind/shell effect will be discussed later in the paper.  Normally, the hydrogen gas which remains on the stellar surface would be pushed away from the star's core as the thermonuclear reaction proceeds and the reaction rate would diminish or self-regulate due to the loss of fuel and decrease in temperature.  The gravity of the white dwarf is so great that the heat generated by the hydrogen burning does not  expand the hydrogen.  Instead, the hydrogen is held close to the star's surface where it continues to burn at an even more rapid rate.  This process is called degenerate hydrogen burning and is what makes the nova burn so brightly.   

Eventually, the hydrogen on the white dwarf's surface is completely consumed.  If the companion star still has hydrogen to donate, and is still within the Roche limit, the process begins again and the nova recurs after some period.  Hellier suggests that all novae are recurrent and than it is only the mass of the white dwarf and the supply of hydrogen that determine the period of the nova.

 

Dominant Spectral Features

Figure 1 shows four spectra of V475 Sct taken over a 38-day period.  The most prominent feature of these spectra is the Hydrogen alpha [H alpha] peak located at 6563.   This peak is caused by the ionization of the hydrogen on the white dwarf's outer surface.  The heat and the resultant explosion caused by the hydrogen fusion reaction push the ionized hydrogen gas away from the star's surface.  Small amounts of higher atomic weight elements from the star's surface are carried along with the expanding hydrogen gas.  Notice that as time progresses the width of the H alpha peak increases slightly.  The importance of this observation will be addressed later in the paper 

The first spectrum was taken twelve days after maximum brightness and clearly shows the development of Fe II peaks in the 4900 and 5250 bands.  These peaks slowly recede in intensity over the following 38 days.  These Fe II peaks will be referred to again when the nova type is determined. 

Classification of V475 Sct

The t2 of V475 Sct has been shown to be 25 days by photometric examination.  The spectrum of V475 Sct shows definite Fe II lines in the early spectra as seen in Figure 1.  The expansion velocity, which was derived from the spectral peak widths, showed a maximum velocity of 1882 Km/sec., and these peaks are sharp and narrow in appearance.  The Mv for V475 Sct, at 7.66, lies slightly above the maximum predicted by Delle Valle for slow novae but is significantly lower than the maximum of 9 Mv predicted for fast novae.  SIMBAD reports that the galactic latitude of V475 Sct is only -3.95o.  At a distance of 2094 pc, the distance from the nova to the galactic plane would only be 144 pc.  [sin 3.95o x 2094 pc = 144 pc]  Therefore, V475 Sct would be classified as a Slow, Fe II type nova and would be expected to have a progenitor that is a Population II star.  The white dwarf component is likely to be less than 1 M.  However, this particular nova is slightly brighter and lies much closer to the galactic plane than the typical slow nova.

 

Estimating the Distance to the Nova

A standard candle, in the jargon of astronomy, is an object that has a brightness that can be used to determine the distance to that same object.  Della Valle, et al. made multiple observations of recent novae to which the distances were accurately known.(Della Valle 1995)  Thus, they were able to use these novae at known distances as standard candles for determining the distances to other novae.  The Della Valle paper developed a formula to determine the distance to a nova as a function of the time that it takes for the nova to lose two apparent magnitudes of brightness from its maximum value.  See formula 2.   Using the Della Valle formula it is possible to estimate a nova's absolute magnitude by knowing how long it takes for the nova to drop two magnitudes from it's maximum brightness.  The absolute magnitude of a star is the brightness of the star if it were observed from a distance of 10 parsecs (pc).  A parsec equals 3.26 light years [ly].   The apparent magnitude of a star is the magnitude that is perceived by an observer on Earth.  The Della Valle relationship is:

Formula 2

Mv = -7.92 - 0.81 arctan (1.32 - log (t2) / 0.23)

 

  Where Mv equals the absolute V band magnitude of the nova and t2 is the time in days for the nova to drop exactly two magnitudes in brightness in the V band.  In the case of V475 Sct, the t2 time was 25 days.  Therefore, the absolute V magnitude of the nova is calculated to be -7.66.   

Once the absolute magnitude of the nova has been determined, the distance to the nova can be calculated by using the

Formula 3

mv - Mv = 5 log (D) - 5 +Av

 

 Where mv equals the apparent or observed magnitude of V475 Sct, D equals the distance to the nova in thousands of parsecs [Kpc] and Av equals the galactic absorption of the light path for the nova in V band magnitudes per Kpc.  The factor for galactic absorption was determined from the NED Coordinate & Extinction Calculator.   The galactic absorption is the amount of light that is absorbed due to gas and dust in the galaxy per 1000 parsecs between the object and the observer.  This factor is dependent on whether the angle of observation is through or along the plane of the galaxy.  It is also dependent on the wavelength of the observation since red light is less disrupted by the intervening dust than is blue light.  In the case of V475 Sct, the V band galactic absorption factor is 1.9 V magnitudes / Kpc.  Therefore, the ratio between the whether the angle of observation is through or along the plane of the galaxy.  It is also dependent on the wavelength of the observation since red light is less disrupted by the intervening dust than is blue light.  In the case of V475 Sct, the V band galactic absorption factor is 1.9 V magnitudes / Kpc.  Therefore, the ratio between the Absorption, Av, and the distance, D, should be ~1.9.  This is because for every Kpc distant the nova is from the observer, the brightness should decrease by ~1.9 mag in the V band.  The distance modulus equation is solved until the ratio between Av and D is 1.9.   The D value that fits the equation for V474 Sct is 2.1 Kpc or ~6800 Ly.

 

A more complete explanation of the classification of V475 Sct may be read from the paper, "Photometry, Spectroscopy and Classification of Nova V475 Sct" .  The paper is available in PDF format here.