The 2011 outburst of the recurrent novaT Pyx. Evidence for a face-on bipolar ejection

Abstract

We report on near-IR interferometric observations of the outburst of the
recurrent nova T Pyx. We obtained near-IR observations of T Pyx at dates
ranging from t=2.37d to t=48.2d after the outburst, with the CLASSIC
recombiner, located at the CHARA array, and with the PIONIER and AMBER
recombiners, located at the VLTI array. These data are supplemented with
near-IR photometry and spectra obtained at Mount Abu, India. Slow expansion
velocities were measured (<300km/s) before t=20d (assuming D=3.5kpc). From
t=28d on, the AMBER and PIONIER continuum visibilities (K and H band,
respectively) are best simulated with a two component model consisting of an
unresolved source plus an extended source whose expansion velocity onto the sky
plane is lower than 700km/s. The expansion of the Brgamma line forming region,
as inferred at t=28d and t=35d is slightly larger, implying velocities in the
range 500-800km/s, still strikingly lower than the velocities of 1300-1600km/s
inferred from the Doppler width of the line. Moreover, a remarkable pattern was
observed in the Brgamma differential phases. A semi-quantitative model using a
bipolar flow with a contrast of 2 between the pole and equator velocities, an
inclination of i=15^{\circ} and a position angle P.A.=110^{\circ} provides a
good match to the AMBER observables (spectra, differential visibilities and
phases). At t=48d, a PIONIER dataset confirms the two component nature of the H
band emission, consisting of an unresolved stellar source and an extended
region whose appearance is circular and symmetric within error bars.These
observations are most simply interpreted within the frame of a bipolar model,
oriented nearly face-on. This finding has profound implications for the
interpretation of past, current and future observations of the expanding
nebula.

Full text

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arXiv:1109.4534v3 [astro-ph.SR] 26 Sep 2011
Astronomy & Astrophysics manuscript no. TPyx˙vDef˙astroph
September 27, 2011
c ? ESO 2011
Letter to the Editor
The 2011 outburst of the recurrent nova TPyx.
Evidence for a face-on bipolar ejection.
O. Chesneau1, A. Meilland1, D. P. K. Banerjee2, J.-B. Le Bouquin3, H. McAlister4,5, F. Millour1, S.T. Ridgway6, A.
Spang1, T. ten Brummelaar5, M. Wittkowski7, N.M. Ashok2, M. Benisty8, J.-P. Berger9, T. Boyajian4, Ch. Farrington5,
P.J. Goldfinger5, A. Merand9, N. Nardetto1, R. Petrov1, Th. Rivinius9, G. Schaefer4, Y. Touhami4, and G. Zins3,⋆
1UMR 6525 Fizeau, Univ. Nice Sophia Antipolis, CNRS, Obs. de la Cˆ ote d’Azur, Bvd de l’Obs., BP4229 F-06304 NICE Cedex 4
2Physical Research Laboratory, Navrangpura, Ahmedabad, Gujarat, India
3UJF-Grenoble 1/CNRS-INSU, Institut de Plan´ etologie et d’Astrophysique de Grenoble (IPAG), UMR 5274, Grenoble, France
4Georgia State University, P.O. Box 3969, Atlanta GA 30302-3969, USA
5CHARA Array, Mount Wilson Observatory, 91023 Mount Wilson CA, USA
6National Optical Astronomy Observatories, 950 North Cherry Avenue, Tucson, AZ, 85719, USA
7European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching bei M¨ unchen, Germany
8Max Planck Institut f¨ ur Astronomie, K¨ onigstuhl 17, 69117 Heidelberg, Germany
9European Southern Observatory, Casilla 19001, Santiago 19, Chile
Received, accepted.
ABSTRACT
Aims. TPyx is the first recurrent nova historically studied, seen in outburst six times between 1890 and 1966 and then not for 45
years. We report on near-IR interferometric observations of the recent outburst of 2011. We compare expansion of the H and K band
continua and the Brγ emission line, and infer information on the kinematics and morphology of the early ejecta.
Methods. We obtained near-IR observations of TPyx at dates ranging from t=2.37d to t=48.2d after the outburst, with the CLASSIC
recombiner, located at the CHARA array, and with the PIONIER and AMBER recombiners, located at the VLTI array. These data are
supplemented with near-IR photometry and spectra obtained at Mount Abu, India.
Results. Slowexpansion velocitiesweremeasured (≤300kms−1)beforet=20d.Fromt=28don, theAMBERandPIONIERcontinuum
visibilities (K and H band, respectively) are best simulated with a two component model consisting of an unresolved source plus an
extended source whose expansion velocity onto the sky plane is lower than ∼700km s−1. The expansion of the Brγ line forming
region, as inferred at t=28d and t=35d is slightly larger, implying velocities in the range 500-800km s−1, still strikingly lower than the
velocities of 1300-1600km s−1inferred from the Doppler width of the line. Moreover, a remarkable pattern was observed in the Brγ
differential phases. A semi-quantitative model using a bipolar flow with a contrast of 2 between the pole and equator velocities, an
inclination of i=15◦and a position angle P.A.=110◦provides a good match to the AMBER observables (spectra, differential visibilities
and phases). At t=48d, a PIONIER dataset confirms the two component nature of the H band emission, consisting of an unresolved
stellar source and an extended region whose appearance is circular and symmetric within error bars.
Conclusions. These observations are most simply interpreted within the frame of a bipolar model, oriented nearly face-on. This
finding has profound implications for the interpretation of past, current and future observations of the expanding nebula.
Key words. Techniques: high angular resolution; (Stars:) novae, cataclysmic variables; individual: TPyx; Stars: circumstellar matter
1. Introduction
A classical nova eruption results from a thermonuclear runaway
on the surface of a white dwarf which is accreting material from
a companion star in a close binary system. TPyxidis (TPyx) is
a unique recurrent nova that was in outburst six times between
1890 and 1966 (intervals of ∼20yr). TPyx was discovered in
outburst at a visual magnitude of 13.0 on 2011 April 14.29 UT
(JD=2455665.79); which we take as t0=0 (Waagan et al. 2011).
This is the first outburstofTPyx sinceDecember7,1966,nearly
45 years before.
The evolution of the nova is relatively slow, thereby provid-
ing time and scope for organizingjoint observationswith optical
Send offprint requests to: Olivier.Chesneau@oca.eu
⋆Based on observations made with CHARA at Mount Wilson ob-
servatory and the VLTI at Paranal Observatory under program 287.D-
5012, 287.D-5023, 087.C-0702
interferometry arrays such as CHARA and the VLTI. TPyx is
surrounded by an interesting nebula in expansion that has been
investigated by the HST during more than 10yr (Schaefer et al.
2010, and references therein). The knots are expanding in the
plane of the sky with velocities ranging from roughly500 to 715
km s−1. In contrast, the velocities inferred from Doppler widths
of the ejecta of recent outbursts were observed to be much faster
at about 1500 km s−1. Although TPyx is a well-observed sys-
tem, it still has many mysteries. Why did the ejecta expand so
slowly in the planeof the sky? An importantspectroscopicstudy
of the binary system from Uthas et al. (2010) provided evidence
of a low-inclination for the system orbit (i=10±2◦), a particu-
larly important constraint for the interpretation of interferomet-
ric data, as it appears that the ejecta emitted around these out-
bursting sources are rarely spherical.
This letter presents optical interferometrymeasurements ob-
tained from different facilities which provide important infor-
1

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Chesneau et al.: The 2011 outburst of the recurrent nova TPyx.
Table 1. Journal of interferometric observations.
date MJDt-t0
Phase
2.92
8.81
12.81
13.93
28.76
35.77
48.74
InstrumentBase projected baselines Calibratorsa
2450000.5+
5668.16
5674.04
5678.06
5679.18
5694.00
5701.02
5713.99
length [m]
107.4
45.4/100.3/120.2
74/94/42/113/82/44
202.1/213.3
59.5/95.6/117.7
56.2/89.9/105.6
68/47/63/67/37/40
P.A. [◦]
−872011/04/17
2011/04/23
2011/04/26
2011/04/28
2011/05/12
2011/05/20
2011/06/01
CLASSIC
AMBER
PIONIER
CLASSIC
AMBER
AMBER
PIONIER
W1-W2
K0-A1-I1
A1-G1-I1-K0
W1-E2
UT1-3-4
UT1-3-4
D0-G1-H0-I1
HD78752, HD79290
HD73947
HD78739
HD78752, HD79290
HD73947
HD73947, HD87303
HD78739
−159.0/96.2/ −105.0
−52/−74/−127/81/39/23
−79.4/82.8
127.1/43.1/73.3
136.9/−133/78.6
171/96/132/25/57/180
aCalibrator angular diameters from SearchCal@JMMC(Bonneau et al. 2006): HD78752 (G0V, 0.22±0.02mas), HD79290 (A0V,
0.13±0.01mas), HD73947 (K2III, 0.86±0.02mas), HD87303 (K2III, 0.90±0.07mas), HD78739 (K0III, 0.32±0.02mas).
Fig.1. Light curve of TPyx with the dates of the optical in-
terferometry observations. Blue diamonds indicate a subset of
AAVSO data in V, and green squares in I. Orange triangles and
redstars indicate H andK bandphotometryfromMt Abu(India)
.
mation when included within a common frame of interpretation.
The observations are presented in Sect. 2. In Sect. 3 we analyze
the continuum measurements by means of simple geometrical
model, and the differential observables through the Brγ line us-
ing a simple model and then discuss the results in Sect. 4.
2. Observations
Near-infrared JHK photometric and spectroscopic observations
were obtained on a regular basis from the 1.2m telescope at the
Mt. Abu Observatory,India. These measurements helped to pre-
pare the interferometric observations and to evaluate the rela-
tive contribution of the various continuum and line components
(Fig.1). Initial observations are reported in Banerjee & Ashok
(2011) while a fuller study is in preparation.
Prompt broad-band interferometric observations were se-
cured with CLASSIC, a two-telescope high sensitivity system
located at CHARA on Mt. Wilson (ten Brummelaar et al. 2005).
Despite the faintness and low declination of the source, observa-
tions in the K-band were obtained at t=2.92d (K≈6.4, from Mt
Abu observations) and t=13.93d (K≈5.7). The log of the obser-
vations is presented in Table1 and the data in Fig.2.
Several interferometric observations at medium spectral
resolution (R=1500) across the Br-γ line were obtained
with AMBER, a 3-telescope combiner located at the VLTI
(Petrov et al. 2007). The first observations were performed
with the 1.8m Auxiliary Telescopes (ATs) at t=8.81d, when
Fig.2.
CLASSIC at t=2.92 (green diamond) and t=13.93 (red trian-
gles). The thick dotted and dashed lines indicate are the UD
curves corresponding to Table 2.
K-band interferometric visibilitiesobtainedwith
the source was below the interferometric sensitivity limit of
AMBER (K≈5.7), but a useful spectrum was obtained. The
second and third measurements, obtained with the 8.2m Unit
Telescopes (UTs) at t=28.76 (K≈4.9) and t=35.77d (K≈5), pro-
vided good quality dispersed visibilities, closure and differential
phases (see Fig.5). Unfortunately, the calibrator measurement
for the last date is of poor quality preventing any reliable cali-
bration of the absolute visibility.
Imaging broad-band interferometric observations were ob-
tained at t=12.81d (H≈6) with the PIONIER visitor instrument
(Berger et al. 2010; Le Bouquin et al. 2011). These observations
provided the simultaneous measurement of 6 absolutely cali-
brated visibilities and 4 closure-phases in the H-band, there-
fore allowing the study of the spatial morphology of the near-
infrared emission. A critical second observation was obtained at
t=48.74d (H≈6), again with the ATs (Fig.3).
3. Analysis
The absolutevisibility measurementswere fitted with simple ge-
ometrical models using the LITpro software (Tallon-Bosc et al.
2008, JMMC). The results are shown in Table2. A simple uni-
form disk (UD) model, i.e. a circular disk of uniform bright-
ness in the plane of the sky, was fitted to the measurements for
the early observations. For later observations, a two component
modelconsistingin an unresolvedcomponent,anda co-centered
uniform disk was used. No evidence of asymmetry was detected
in the data.
The first K-band CLASSIC measurement, obtained with a
∼100m baseline was consistent with a weakly resolved source,
while the second set (t=13.93d) with a baseline about twice
2

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Chesneau et al.: The 2011 outburst of the recurrent nova TPyx.
Table 2. Analysis of the V2using geometrical models.
Instrument Spectral Bandt-t0
day
Single component model
UD diameter
[mas]
1.±0.2
0.6±0.1
1.12±0.14
2.58±0.3
2.23±0.1
Double components model
Unres. FluxUD Flux
[%]
-
-
-
65±12
83±9
UD diam.
[mas]
-
-
-
7.3±0.3
8.5±0.2
[%]
-
-
-
35±8
17±2
CHARA/CLASSIC
VLTI/PIONIER
CHARA/CLASSIC
VLTI/AMBER
VLTI/PIONIER
broad K
broad H
broad K
2.1±0.05µm
broad H
2.92
12.81
13.93
28.76
48.74
2011−04−27
2011−06−02
0 20 40 60 80
0.6
0.8
1.0
North
East
−50 0 50
−50
0
50
Baseline (m/µm)
V2 (H−band)
Fig.3.
PIONIER at t=12.81 (open red) and t=48.74 (filled green). The
corresponding uv-plane is displayed in the subpanel.
H-bandinterferometric visibilitiesobtainedwith
as long, shows a resolved source whose size does not seem
to be dramatically changed. The first PIONIER measurements
(t=12.81d)provideda H-bandUD diametersignificantlysmaller
than the CLASSIC K band measurements, even taking into ac-
count the small time difference between the two measurements.
This effect cannot be attributed to emission lines seen at these
dates, that contribute less than 10-15% of the flux in the H band
and less than 5% in the K band. A large scale component with
a rising flux contribution in the K band may account for the ob-
servations, and is consistent with a H-K flux difference of 0.33
mag that is observed consistently during the event. Assuming
a two component model, with a fixed contribution of 70±10%
from a ’stellar’ source with a diameter of 0.6mas, the extended
K-band source should have a diameter larger than 1.5-2mas,and
thereforebealmostfullyresolvedbyCLASSIC.Assumingadis-
tance of D=3.5±1kpc from Schaefer et al. (2010), that may be a
lower limit (Shore et al. 2011), the expansion velocity inferred
from such an extendedcomponentis about 500kms−1, while the
H-band core expansion is estimated to be ∼100km s−1(Fig.4).
Interestingly,the FWHM of theBrγ measuredfromthe AMBER
spectrum at t=8.81d is 590km s−1, i.e. is consistent with our hy-
pothesis of the expanding extended component (Fig.5).
Then follows a second epoch during which AMBER inter-
ferometric data were obtained on two dates, yet providing cal-
ibrated visibilities only at t=28.76d. A single UD does not ac-
count well for the observed visibilities at t=28.76d (reduced
χ2
data (χ2
extended continuum component of 700km s−1. This is in con-
trast with the width of the Brγ line, for which we measured a
FWHM of 1050±50km s−1. Furthermore, the Doppler veloc-
ity associated with the P-Cygni absorption in the line is found
to be −1450±100km s−1(Fig.5). The Brγ line-forming region
r=35), the two component model provides a better fit to the
r=5). This gives an upper limit for the expansion of the
Fig.4. Result of the Uniform Disk (UD) estimates from the
continuum V2measurements from the various interferometers.
Before t=20d, the source size is estimated using a single UD in
the H and K bands (PIONIER: orange triangle; CLASSIC: red
diamonds). The last points indicate the extended source in the
double component model (AMBER: red square; PIONIER: or-
ange triangle)
should therefore expand much faster, and a large visibility drop
should be measured through the Brγ line, as seen for RSOph
(Chesneau et al. 2007). However, the dispersed Brγ visibilities
are only slightly lower than the nearby continuum, implying a
moderatediameterincreaseoflessthan10%.Oneweeklater,the
visibilities droppedin the line indicatinga largeexpansionof the
Brγ line-forming region (taking into account the 25% increase
of the line flux). At that time the line FWHM was measured
to be 1600±50km s−1and the P-Cygni absorption indicated a
wind velocity of 1800±100kms−1. The differential phases show
a complex structured signal that can be described by 2 opposite
S-shaped signals, with variations related to the baseline lengths
and P.A. dependency. The pattern is symmetrical about the line
center, with the width and amplitude of the signal increasing be-
tween the two dates, although the amplitude never went beyond
10◦(again by contrast with what was observed for RSOph).
Thelastobservations, performed at
PIONIER, bring complementary and crucial information,
owing to the larger uv coverage involved. The H band source is
now well resolved and departs from a simple model. A good fit
(χ2= 1.1) is reached using the two component model (Table2).
The extended component estimated expansion velocity in the
plane of the sky is lower than 700km s−1. The closure phases
do not exceed 2.5◦. Moreover using a flattened structure for the
extended component does not improve the fit and constrains the
aspect ratio to 1±0.07. This implies that the complex yet weak
t=48.74d with
3

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Chesneau et al.: The 2011 outburst of the recurrent nova TPyx.
Fig.6. Brγ Bipolar-flow model (without central source) seen at
i=90◦(right) and i=10◦(left , the best model) and P.A.=110◦
(best model).
phase signal seen by AMBER originates from a source with a
predominantly symmetrical appearance.
4. A face-on bipolar event
The heterogeneous data set originating from different instru-
mentsprovidesintriguingdata,andis uniquein viewofthe com-
plementary constraints providedfor the analysis. The interpreta-
tion of these data can be divided into several key temporal steps.
– CLASSIC and PIONIER data obtained at t=2.92d, 12.81d
and t=13.93d show an extended H and K band source.
The fact that TPyx is resolved so early is intriguing. An
hypothesis might be a light echo witnessing a close-by
circumbinary environment, adding incoherent flux to the
measurement. The expansion rate is then low between the
first and second measurements, and a striking difference is
observed between the H and the K band inferred diameters.
This suggests an advanced decoupling between a shrinking
optically thick core and an expanding free-free emitting
optically thin envelope.
– The AMBER data obtained at t=28.76d provide evidence
that the Brγ line forming region projected onto the sky is
formed close to the expanding continuum, while being more
extended at t=35.77d. The Doppler velocity accelerated
from 1300km s−1to 1600km s−1in the time interval. A
striking differential phase pattern is observed.
– The PIONIER dataset secured at t=48.74d is critical for es-
tablishing the two component nature of the emission, con-
sisting in an unresolved stellar source and an extended re-
gion whose appearance is circular and symmetric within er-
ror bars.
A face-on bipolar event could account for the ensemble of
informationdescribed above,in accordancewith the finding of a
low inclination for the system (Uthas et al. 2010) . In particular,
the differential phase pattern can be linked to the geometry and
kinematics of the ejecta. We developed a ‘toy’ model that pro-
vides a good match to the observations. Given two ad-hoc three-
dimensional distributions, one for the ‘emission’ of the ejecta
and one for the velocity field, we reconstructed intensity maps
in narrow spectral bands in the emission line and then computed
the corresponding visibilities and differential phases. The inten-
sitymapiscreatedconsideringthatthematterwas ejectedduring
a brief outburst (best ascribed by a shell), propagating at vr(θ).
Consequently, the geometry is directly related to kinematics of
the ejecta. We used the following radial expansion law:
vr(θ) = vpole+
?
veq− vpole
?
sinθ
(1)
where θ is the colatitude, and vpole and veq are the polar and
equatorial radial velocities, respectively.
We also considered an emission decreasing according to a
power law of the distance. At a given epoch t the 3D intensity
distribution is proportional to:
I(r,θ,φ) ∝1
rαexp
?−(vrt − r)2
2σ2
r
?
(2)
Using this model we were able to fit the Brγ differential visi-
bilities and phases, as well as the line profile for the two epochs.
The parameters of the best model are: i=15◦, P.A. of the polar
axis of 110◦, α=2 and the velocities for the two epochs:
– vpole=1200kms−1and veq=600kms−1at t=28.76d
– vpole=1600kms−1and veq=700kms−1at t=35.77d
The fit of the differential phases at the two epochs and
the model images are presented in Fig 5 and the model in
Fig 6. The polar and equatorial velocities are in good agreement
with the Doppler and sky plane velocities estimated in Sect.3.
Furthermore, the P.A. of the equatorial plane overdensity is ori-
ented in a direction similar to the P.A. of the faint X-ray nebula
(Balman 2010).
The face-on bipolar nebula allows one to better understand
the curious nebula scrutinized with HST (Schaefer et al. 2010;
Shara et al. 1997, 1989). The knots are concentrated in a ring
(3.2-6”), expanding radially with a velocity in the restricted
range of 500-700km s−1, and with a mean radial velocity of
about500kms−1(O’Brien & Cohen1998).Decipheringbetween
a projected sphere and a bipolar structure producing a dense,
face-onringis difficult,consideringthat radialvelocitymeasure-
ments of individual clumps are missing.
Some recent examples suggest that bipolarity in the ejecta
of classical/recurrent novae may be relatively frequent: RSOph
(Ribeiro et al. 2009; Bode et al. 2007; Chesneau et al. 2007),
V445Pup (Woudt et al. 2009), V1280 Sco (Chesneau et al.
2008, Chesneau et al., in prep.) or HRDel (Harman & O’Brien
2003). A significant difference though exists between the TPyx
and RSOph environments: the lack of material around TPyx,
witnessedforinstancebythelackofhardX-rays(Kuulkers et al.
2011),leadsus tofavora bipolarityinducedbya processinternal
to the system, whetherby the commonenvelopeinteraction with
the companion, since the development of the event is relatively
slow, or by invoking an intrinsically bipolar ejection related to a
spun-up central star (Porter et al. 1998; Lloyd et al. 1997).
Acknowledgements. The CHARA Array is funded by the National Science
Foundation through NSF grant AST-0908253, by Georgia State University, the
W. M. Keck Foundation, the Packard Foundation, and the NASA Exoplanet
Science Institute. Research at the Physical Research Laboratory is funded by the
Dept. of Space, Govt. of India. STR acknowledges partial support from NASA
grant NNH09AK731.
4

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Chesneau et al.: The 2011 outburst of the recurrent nova TPyx.
Fig.5. AMBER data. Top: comparison between the Brγ line at t=8.81 (blue curve), t=28.76d (red curve) and t=35.77d (green
curve).Bottom: Above - differential visibility comparisonbetween t=28.76dand t=35.77d(scaled to the continuumV2at t=28.76).
Below: same with differential phases. The phases are compared with the phases from the model (dashed line, χ2
respectively).
r=1.1 and 1.4,
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