A&A 452, 567-569 (2006)
DOI: 10.1051/0004-6361:20054720
U. Munari1 - A. Siviero1 - H. Navasardyan1 - S. Dallaporta2
1 - INAF Osservatorio Astronomico di Padova, via dell'Osservatorio 8, 36012 Asiago (VI), Italy
2 -
via Filzi 9, 38034 Cembra (TN), Italy
Received 19 December 2005 / Accepted 7 March 2006
Abstract
Our CCD photometry of Nova Scuti 2005 N.2 (=V477 Sct)
shows it to be a very fast nova, which is characterized by t2 = 3 and t3 = 6 days, affected by a EB-V
1.3 mag reddening, and which peaked at
V
9.8 mag on
Oct. 12.0 UT.
The nova was probably entering a dust condensation episode or
brightness oscillations during the transition phase when it became
unobservable for the seasonal conjunction with the Sun. Absolute
spectrophotometry shows it to belong to the He/N class. The emission
line width at half intensity is 2600 km s-1. At least five ripples are
identified in the high resolution emission lines profiles at radial
velocities ranging from -980 to +700 km s-1. The nova erupted at a large distance from the Sun and at an appreciable height above the Galactic plane, suggesting an association with the Galactic bulge (unusual for a He/N nova). The progenitor was too faint to be
recorded on DSS1/2 survey plates, when setting the outburst amplitude to
11 mag.
Key words: stars: novae, cataclysmic variables
Nova Scuti 2005 N.2 (=V477 Sct) was discovered by Pojmanski (2005) on
ASAS patrol images, shining
at V = 12.0 on Oct. 11.026 and at V = 10.4 on Oct. 13.066 (UT) indicating
that the nova was first caught during the rise to maximum. The nova was
independently discovered by Haseda (2005). An accurate astrometric position
was derived by Puckett (2005) as
= 18 38 42.93,
= -12 16 15.6 (corresponding to galactic coordinates l = 20.57,
b = -2.79). No field star is visible at this position on DSS1 and DSS2 survey plates, indicating an outburst amplitude
11 mag. The
absence of the progenitor on the 2MASS survey excludes its belonging to the
class of recurrent novae with a cool giant donor star (like T CrB or RS Oph).
Very little is known about this nova that was discovered shortly before
becoming lost in the seasonal conjunction with the Sun. Das et al. (2005)
report that on Oct. 15.75 (UT) the nova displayed prominent H I emission
lines of the Paschen and Brackett series on infrared spectra (1.08-2.35 m range), indicating an FWZI of 6000 km s-1, while an optical spectrum on
16.43 (UT) by Fujii (2005) shows a reddish continuum with broad emission
lines including H
,
H
,
and O I 7773 Å characterized by an FWHM of 2900 km s-1.
Low and medium resolution spectra of Nova Scuti 2005 N.2 were secured on Oct. 27.7 with the AFOSC imager+spectrograph mounted on the 1.82 m telescope operated
in Asiago by INAF Astronomical Observatory of Padova. We obtained absolutely
fluxed low-resolution spectrophotometry covering the range 3505-7815 Å with a dispersion of 4.2 Å/pix. The spectrum is presented in Fig. 1,
with line identification superimposed. Higher resolution emission line profiles
were obtained with holographic grisms over short wavelength intervals covering H
and OI 8447 Å lines (6390-7045 Å at 0.6 Å/pix, and
8265-9165 Å at 0.9 Å/pix, respectively). The velocity profile of both lines
is presented and compared in Fig. 2. The spectra can be obtained in electronic
form from http://ulisse.pd.astro.it/novasct2005n2/ and from the CDS.
The CCD B, V,
photometry on Nova Scuti 2005 N.2 was secured from a private observatory near Cembra (Trento), Italy, housing a 28 cm
Schmidt-Cassegrain telescope. The data reduction was performed in a standard
fashion in IRAF. The photometric data are reported in Table 1, and the
photometric evolution of the nova is presented in Fig. 3. The photometric
data are calibrated on nearby TYC 5700-812-1 used as a comparison star
(being present in the same frames as the nova) for which we adopted
= 10.96,
= 10.27,
= 9.50. Johnson's
and
are derived from Tycho-2's
and
following
Bessell (2000) transformations.
is derived from Johnson's
and
following Caldwell et al. (1993) transformations.
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Figure 1: The absolutely fluxed 3500-7800 Å low resolution spectrum of Nova Scuti 2005 N.2 observed on 2005 Oct. 27.76 UT. Most probable identification for the emission lines is given. The ordinate scale is logarithmic to emphasize the visibility of weaker features. |
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Figure 2:
Resolved H![]() |
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The lightcurve in Fig. 3 is clearly that of a very fast nova. The exact maximum is somewhat uncertain, there being no data between Oct. 11.026, when the nova was discovered on the rising branch at V = 12.0, and Oct. 13.066 when it was at V = 10.4 and fading already. A reasonable hand-drawn fit of the early light-curve in Fig. 3 suggests that the maximum was reached on Oct. 12.0 (UT) at V = 9.8 mag, so for sake of discussion we adopt this in the following.
The nova decline times are t2 = 3 and t3 = 6 days, making it one of the
fastest known novae. Among the very few known faster classical novae, there
are Nova Her 1991 (t2 = 1.2 and t3 = 3.2 days) and Nova Ser 1983 (t2 = 2
and t3 = 5 days). Using the relation between t2 and the outburst amplitude
of Warner (1995), the nova progenitor should have shined at V
23, so it is
no surprise that it does not show up on DSS1 or DSS2 plates. The color of the
nova remained stable at B-V = 1.3 for the first three weeks (cf. bottom
panel of Fig. 3), which
suggests a reddening amounting to EB-V = +1.3 after adopting
an intrinsic (B-V)0 = 0.0 at time t2 from Warner (1995). This agrees with the
Neckel & Klare (1980) extinction maps that indicate EB-V
1 along
the line of sight to the nova for any reasonable distance estimate. The
equivalent width of the diffuse interstellar band at 6614 Å (barely
noticeable at the compressed scale of Fig. 2 on the red wing of the
H
profile) is 0.16(
0.02) Å, much larger than the 0.04 Å observed by Herbing (1995) in HD 183143, which is similarly reddened by
EB-V = +1.3, thus confirming a high reddening affecting Nova Sct 2005 N2,
probably even larger than EB-V = +1.3.
Table 1:
BV
CCD photometry of Nova Scuti 2005 N.2.
MJD
= heliocentric JD - 2 450 000.
Using the Cohen (1988) relation between absolute magnitude and t2 rate of
decline, the nova absolute magnitude is =
-9.5 (an identical value is found using the Schmidt (1957) relation for t3), which
corresponds to a distance of 11 kpc, and a height above the Galactic plane
z
0.6 kpc for EB-V = +1.3 and
= 9.8 mag.
Propagating the uncertainties involved with absolute and observed magnitude
at maximum and with reddening, the error budget on distance and z is 33%,
allowing an association of the nova with the Galactic bulge.
The drop in magnitude indicated by the last photometric point in Fig. 3
suggests that the nova was at that time entering a dust condensation episode
in its ejecta or that it was beginning to experience the oscillations
sometimes seen during the transition phase of fast novae. The color at that
time
= +3.6 was significantly redder than the stable mean value
+3.0 characterizing the previous part of the
lightcurve. However, this is not a conclusive argument in favor of a dust
condensation episode because color changes have sometimes been observed to
accompany oscillations, too. Quite unfortunately, bad weather and
conjunction with the Sun prevented us from extending the observations in
time to properly cover this interesting phase of the lightcurve.
Integrating the B and V magnitudes on the spectrum of Fig. 1 using Buser (1978)
transmission profiles provides B = 15.36(0.05) and
V = 14.04(
0.04) mag, which fit the photometric evolution in
Fig. 3 well (where they are represented by the cross symbol), thus enforcing
confidence in the accuracy of the calibration into absolute fluxes of the spectrum.
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Figure 3: The photometric evolution of Nova Scuti 2005 N.2. The dots mark our CCD observations in Table 1, the crosses correspond to B, V band integration over the absolutely fluxed spectrum of Fig. 1, and the open circles are values published in IAUC 8617. The dashed line is hand drawn. |
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Table 2:
Radial velocity of the ripples numbered on the H
and
OI 8447 Å emission line profiles of Fig. 2.
The spectrum in Fig. 1 shows that the nova had not yet entered the nebular
phase at the time of observation (+16 days from maximum). The spectrum
lacks significant FeII lines and is instead rich in He and N lines, allowing
us to associate Nova Scuti 2005 N.2 with the He/N class defined by Williams
(1992), which is consistent with the very fast speed class of the
nova. It is worth noticing that
Della Valle & Livio (1998) found a typical scale
height of He/N novae above the Galactic plane of
100 pc, significantly less than the z
0.6
0.2 kpc we have
derived above for this nova. The spectrum in Fig. 1 bears some
resemblance to the spectrum of Nova LMC 1990 N.1 presented by Williams et al. (1991), which at later stages evolved into a Neon nova.
The high resolution profiles of H
and OI 8446 Å in Fig. 2
display a width at half intensity of 2645(
15) and 2590(
15) km s-1,
respectively, within the observed spread of the McLaughlin (1960) relations
between expansion velocity and t2, t3 decline rates. These velocities
are about 10% slower than found by Fujii (2005) for observations secured 11 days earlier, in agreement with the expected velocity decrease with time
(e.g. Warner 1989).
Both the profiles in Fig. 2 display a series of ripples. Their radial velocities are given in Table 2. There is good correspondence in the radial velocity of the same ripple observed in the two distinct profiles, supporting a real kinematic identity. Such ripples can be ascribed to large, distinct blobs of material ejected by the nova at different angles with respect to the line of sight (beautifully visible in the HST images of Nova Cyg 1992 and T Pyx). Alternatively, they can be caused by projection effects of equatorial and polar rings of enhanced brightness in the expanding ejecta as shown in the atlas of computed lines profiles by Gill & O'Brien (1999).