RS Puppis: a Unique Cepheid Embedded in an Interstellar Dust Cloud

Astronomical Science

RS Puppis: A Unique Cepheid Embedded in an Interstellar Dust Cloud

Pierre Kervella1 remnant of the interstellar material from on the three-dimensional distribution of Antoine Mérand2 which the Cepheid formed. the scattering material, and the position László Szabados3 of the observer (see, e.g., Sugerman William B. Sparks4 [2003] for details). Figure 2 shows a sche- Alexandre Gallenne5 Light echoes: a stimulating geometrical matic view of the propagation of the Robert J. Havlen6 puzzle maximum light wavefronts emitted by RS Howard E. Bond4 Pup into space. The geometrical shape Emanuela Pompei2 Cepheids are historically the most im of these wavefronts is almost a parabo- Pascal Fouqué7 portant class of variable stars, thanks to loid (rigorously, they are very elongated David Bersier8 the relation between their pulsation pe ellipsoids). Their intersection with a thin Misty Cracraft4 riod and absolute luminosity; that relation light-scattering dust layer produces “light played a key role in the discovery of the rings”, such as those visible in Figure 1 expansion of the Universe. In particular, (right). For the nebula of RS Pup, the 1 LESIA, Observatoire de Paris, UPMC, long-period Cepheids are so bright that irregular distribution of the dust results in Université Paris Diderot, France they can be observed in very distant imperfectly circular rings. 2 ESO galaxies, and act as standard candles to 3 Konkoly Observatory, Hungary measure their distances. With its 41.4- Light echoes are particularly interest- 4 Space Telescope Science Institute, USA day pulsation period, RS Pup is one of ing as they may provide a means of 5 Universidad de Concepción, Chile the brightest known long-period Cepheids measuring the geometrical distance to 6 307 Big Horn Ridge Dr., Albuquerque, in the Galaxy. But this star also stands the central star. For a Cepheid like RS USA out as the only known example of the inti- Pup, such an independent distance 7 IRAP, Université de Toulouse, France mate association of a Cepheid with a measurement is of great interest for the 8 Astrophysics Research Institute, dust cloud (Figure 1, left). The discoverer calibration of the Period–Luminosity rela- Liverpool John Moores University, UK of the nebula of RS Pup, Bengt Westerlund tion. From imaging of the light echoes (Westerlund, 1961), has already remarked with the ESO Multi-Mode Instrument on that “the nebula may be large enough the New Technology Telescope (NTT/ The long-period Cepheid RS Pup illu to permit the detection of possible varia- EMMI), Kervella et al. (2008) derived its minates its circumstellar dust cloud tions in its intensity distribution due to distance based on the assumption that with a variable flux, and creates spec the light variations of the star”. The exist- the observed nebular features are located tacular light echoes. Through photo ence of this “light echo” phenomenon, close to the plane of the sky. However, metric observations of this phenome created by the propagation of the lumi- as argued by Bond & Sparks (2009), this non with NTT/EMMI, and polarimetric nosity variations of the star in the nebula, simplifying assumption could lead to a imaging with VLT/FORS, we have was subsequently confirmed by Havlen significant bias on the determined dis- derived the geometric distribution of (1972). tance. To derive the distance of RS Pup the scattering dust and the total mass unambiguously using its light echoes, of the nebula. We conclude that the The geometrical configuration of the light we therefore need first to determine the dust was not created by mass loss from echo phenomenon is somewhat counter- three-dimensional geometry of the dust the Cepheid, but is most probably a intuitive, as its morphology depends both nebula.

Ma ximu Minimu m ligh t m ligh Towards the Earth 0 t Z r R

RS Pup y sk

Figure 1. Left: Three-colour BVR e th composite image of the nebula of RS e of

Pup obtained using the NTT/EMMI an Pl instrument. Upper: Ratio of two Light–scattering dust layer images obtained a few days apart, showing the light echoes (from Kervella et al., 2008). Figure 2. Geometry of the propagation of the maximum and minimum light wavefronts in space, for an observer located to the left of the figure.

46 The Messenger 150 – December 2012 The first piece of information on the dust 7.5 of the degree of linear polarisation over geometry is provided by the high contrast the nebula (Figure 4, upper right). The of the light echoes in the nebula. The altitude Z of the dust layer relative to the 8.0 relative amplitude of the photometric vari- plane of the sky is obtained in the same ation on any part of the nebula is compa- e physical units as the projected distance rable to the variation of the central star 8.5 R. As a consequence, to obtain the alti-

itself. As shown in Figure 3, the amplitude magnitud tude in absolute linear units, we have to of the luminosity variation of RS Pup is B assume the distance of the star to be relatively large, particularly in the B-band 9.0 1.8 ± 0.1 kpc (estimated from indirect (there is almost a factor of five between techniques). The resulting altitude map is the minimum and maximum luminosity). 9.5 presented in Figure 4 (lower left). The fact that this high amplitude is pre- –0.2 0.0 0.20.4 0.6 0.81.0 1.2 served in the scattered light indicates that Phase The light-scattering material surrounding the dust is confined in a geometrically Figure 3. The light curve of RS Pup in the B-band RS Pup appears to be spread over an thin veil, as a thick layer would result in (data from Berdnikov et al. [2009]). irregular surface, with no well-defined the out-of-phase superposition of differ- central symmetry relative to the Cepheid. ent wavefronts, and therefore the disap- on the Very Large Telescope (VLT/FORS; The visual shape of the nebula (Figure 4, pearance of the echoes. Kervella et al., 2012). The resulting polari upper left) appears more symmetric, metric images allowed us to derive a map due to the higher efficiency of forward To determine the spatial shape of this dust layer, we used the particular linear polarisation signature of the scattering of light by dust grains. The degree of linear polarisation ρ of the scattered light is linked to the scattering angle θ, i.e., the angle between the incident and emergent directions of the photon (see Figure 2). The maximum polarisation is ~ 50% for a scattering angle of θ = 90°, i.e. for scattering material located in the plane of the sky, and a zero polarisation degree is obtained for forward or backward scattering (θ = 0 or 180°). We used the empir- ical calibration of the ρ(θ) relation ob tained by Sparks et al. (2008) from the light echo of the cataclysmic variable star

V838 Mon. This relation is very close 015002000 1500 000 2500 3000 3500 00.050.1 0.15 0.20.250.3 0.35 0.40.45 to the theoretical scattering law for very small dust grains (Rayleigh scattering). From a measurement of ρ, we can therefore retrieve the scattering angle θ.

The projected angular separation R is directly measured in the images, and by combining R and θ, we can estimate the altitude of the scattering material Z = R/tan(θ) above the plane of the sky (i.e. the imaginary plane orthogonal to the line of sight, located at the distance of RS Pup). It should be noted that backward scattering is much less efficient than forward scattering, and we therefore observe essentially the material located between

RS Pup and us, while the dust located 0.20.3 0.40.5 0.6 0.70.8 0.91 01020304050607080 behind RS Pup will be very much fainter. Figure 4. Upper left: FORS+EMMI combined intensity front of RS Pup, relative to the plane of the sky (in To measure θ for each point of the nebula, image of the nebula of RS Pup in the V-band (field parsec). Lower right: Density of the scattering mate- of view 4 by 4 arcminutes). Upper right: Map of the rial (in units of 1054 H atoms per square arcsecond, we took advantage of the polarimetric degree of linear polarisation measured with FORS. i.e. the equivalent number of hydrogen nucleons in imaging mode of the FORS spectrograph Lower left: Altitude of the light-scattering dust layer in the scattering material). From Kervella et al. (2012).

The Messenger 150 – December 2012 47 Astronomical Science Kervella P. et al., RS Puppis: A Unique Cepheid Embedded in an Interstellar Dust Cloud

Figure 5. Images of the nebula surrounding RS Pup in the V-band (EMMI, left), and Spitzer infrared images at 24 μm (middle) and at 70 μm (right), on the same spatial scale. The position of the Cepheid is marked with a star symbol (from Kervella et al., 2009).

N 100 000 au 1 arcmin E scattering that tends to emphasise the this figure only the material that is illumi- the scarcity of similar Cepheid–nebula material located close to the line of sight. nated by RS Pup within 1.8 arcminutes associations, and also confirms that the High degrees of linear polarisation from the star. FORS and EMMI long ex presence of such nebulae is probably not (~ 50%) are observed for several nebular posures show faint nebular extensions far a significant source of photometric bias knots, showing that their position is close beyond this region, at least up to a radius for the calibration of the Cepheid Period– to the plane of the sky (represented in of 3 arcminutes. These distant parts are Luminosity relation. Our determination of blue in Figure 4, lower left). too faint to measure their degree of linear the three-dimensional distribution of the polarisation. dust will also allow us to use the phase of the light echoes to determine unambigu- Origin of the dust nebula This determined mass corresponds only ously the distance of RS Pup. This is a to the light-scattering dust, but does particularly important piece of information As we have determined the geometry of not include the gaseous component, that for this rare, long-period Cepheid, which the dust layer, it is now possible to com- is transparent to light. The typical gas- is a typical example of the stars used to pute the physical distance r between each to-dust mass ratio in the Galaxy is ~ 100. determine extragalactic distances. point in the nebula and the Cepheid (r2 = Z2 The total mass of the nebula, including + R2, Figure 2). Knowing r and the scat- the gas, is therefore approximately 300 As RS Pup illuminates the nebula, part tered light flux, we can deduce the local solar masses. Such a high mass is clearly of its radiation is absorbed by the dust, density of the scattering material at any incompatible with the scenario that RS which warms up and emits radiation location on the nebula. Figure 4 (lower right Pup created the nebula through mass loss by itself. As the dust is still rather cold panel) shows the resulting density map. (the mass of the Cepheid itself is esti- (~ 40–60 K; Barmby et al., 2011), this

mated to be ~ 13 MA). Most of the dust emission occurs in the infrared domain, Compared to Figure 4 (upper left), the observed in scattered light therefore but it is clearly observable in the Spitzer highest densities do not generally cor appears to be of interstellar origin. The images presented in Figure 5. With respond to the brightest parts of the neb- higher density dusty environment in which observations of both the scattered light in ula. This is easy to understand as the RS Pup is presently located could either the visible, and the thermal emission in scattered light flux depends both on the be the remnant of the molecular cloud the mid-infrared domain, together with scattering angle θ and the linear distance from which the star formed, or unrelated the three-dimensional map of the dust r between the star and the dust. A par- interstellar material into which RS Pup is distribution, RS Pup is a particularly ticularly prominent feature is the “ridge” temporarily embedded, due to its motion promising test case for the study of inter- structure visible south of RS Pup. This fil- in the Galaxy. The thin veil geometry of stellar dust properties. amentary dust cloud is located close to the dust layer is probably created by the the plane of the sky, with a slight inclina- radiation pressure from the extremely References tion, as shown in Figure 4 (lower left). bright Cepheid (~ 17 000 LA), that sweeps the interstellar dust away from the star. Barmby, P. et al. 2011, AJ, 141, 42 From the density map, assuming standard Berdnikov, L. N. et al. 2009, Astron. Lett., 35, 406 interstellar dust properties, we estimate Bond, H. E. & Sparks, W. B. 2009, A&A, 495, 371 the total mass of the light-scattering dust Prospects Havlen, R. J. 1972, A&A, 16, 252 to be 2.9 ± 0.9 M . This value should Kervella, P. et al. 2008, A&A, 480, 167 A Kervella, P., Mérand, A. & Gallenne, A. 2009, A&A, be regarded as a lower limit to the true Thanks to our EMMI and FORS observa- 498, 425 mass of the nebula, as we sample only tions, we have established that RS Pup Kervella, P. et al. 2012, A&A, 541, A18 the material in front of RS Pup, while dust did not create the nebula in which it is Sparks, W. B. et al. 2008, AJ, 135, 605 is likely to be present also behind the currently embedded. This peculiar config- Sugerman, B. E. K. 2003, AJ, 126, 1939 Westerlund, B. E. 1961, PASP, 73, 72 plane of the sky. In addition, we include in uration provides a natural explanation to

48 The Messenger 150 – December 2012