The circumstellar envelopes of the Cepheids $\mathsf{\ell}$ Carinae and RS Puppis

Abstract

Context. Compact circumstellar envelopes (CSEs) have been detected around several Cepheids by interferometry. In particular, observations with the VINCI instrument revealed in 2006 a compact envelope around the long period Cepheid Car (P = 35.5 days). Another Cepheid, RS Pup (P = 41.4 days), presents a large (≈2 ) circumstellar nebula scattering the Cepheid light in the visible. These envelopes are particularly interesting for two reasons: their presence could impact the Cepheid distance scale, and they could imply stellar mass loss. Aims. Our goal is to establish the spatial and spectral properties of the CSEs of Car and RS Pup. This is done through a parametrization of the envelopes in terms of fractional flux (with respect to the star) and angular size. Methods. We retrieved archival Spitzer images of the two stars (λ = 3.5−70 μm), and obtained new diffraction-limited imaging with the VLT/VISIR camera in BURST mode (λ = 8.6−11.9 μm), as well as interferometric observations with the VLTI/MIDI beam combiner (λ = 8−13 μm). This combination of single-telescope and interferometric techniques allows us to probe the envelopes of the two Cepheids over a broad range of angular scales, from arcminutes to milliarcseconds. Results. The circumstellar envelope of RS Pup is resolved spatially at 24 and 70 μm by Spitzer, and around 10 μm by MIDI and VISIR. The envelope of Car appears much more compact, and is resolved only in the VISIR and MIDI observations. The infrared excesses we detect around RS Pup and Car are both very significant, but differ considerably in spectral and spatial properties. We detect a warm component in the CSE of both stars at a spatial scale of a few 100 to a few 1000 AU. In addition, RS Pup presents a very large (several 100 000 AU) and cold (≈40 K) dusty envelope. Conclusions. The observed properties of the CSEs lead us to propose that the cold dust content of the large reflection nebula surrounding RS Pup has an interstellar origin, while the warm CSEs of the two stars were created by ongoing stellar mass loss. We also speculate that the NGC 7023 reflection nebula surrounding the Herbig Be star HD 200775 is an analogue of RS Pup at an age of 100 000 years. The presence of CSEs around the two brightest long-period Cepheids is an indication that many long-period members of this class, if not all, could be surrounded by warm circumstellar envelopes created by mass loss. However, very large dusty envelopes such as that of RS Pup are probably less common, as according to our scenario, they require the presence of a high dust density in the interstellar medium (ISM) at the time of the formation of the Cepheid progenitor.

Extended envelopes around Galactic Cepheids

“…This effect can be interpreted as emission at longer wavelengths coming from cooler material that is located at larger distances from the Cepheid than the warmer material emitted at shorter wavelengths. Kervella et al (2009) previously observed the same trend for Car and RS Pup.…”

“…Our previous works (Gallenne et al 2011;Mérand et al 2007Mérand et al , 2006; Kervella et al 2006) showed that these CSEs have an angular size of a few stellar radii and a flux contribution to the photosphere ranging from a few percent to several tens of percent. While in the near-IR the CSE flux emission might be negligible compared with the Based on observations made with ESO telescopes at Paranal observatory under program ID 082.D-0066 Table 3 is only available in electronic form at http://www.aanda.org photospheric continuum, this is not the case in the mid-and far-IR, where the CSE emission dominates (Gallenne et al 2011;Kervella et al 2009). …”

The long-period Galactic Cepheid RS Puppis

“…The lack of central symmetry in the nebula also points at an interstellar origin for the nebular material. Following Kervella et al (2009), we propose that the nebula is either a remnant of the interstellar cloud in which RS Pup formed, or a particularly dense interstellar cloud in which it is temporarily embedded due to its proper motion. There are indications in the nebula geometry that the Cepheid participated in the shaping of the dust cloud, e.g.…”

“…As discussed previously, mass-loss probably happened during the RSG phase that took place in the recent evolution of the star. In addition, Kervella et al (2009) identified a compact and hot circumstellar envelope around RS Pup, that is likely due to presently ongoing mass loss. This also supports the findings of Deasy (1988), who identified a very significant mass loss rate for RS Pup (Ṁ = 10 −6 M yr −1 ), based on IUE spectra.…”

VEGA: Visible spEctroGraph and polArimeter for the CHARA array: principle and performance

“…Compared to interferometric observations in the infrared, the application of the interferometric Baade-Wesselink method will be more efficient using visible angular diameters from VEGA, as the complementary spectroscopic velocimetry is also obtained in the visible. The high spectral resolution will also make it possible to study the atmosphere dynamics in metallic lines (Nardetto et al 2007), as well as the signatures of their envelopes in hydrogen lines (Nardetto et al 2008;Kervella et al 2009). …”