Near-infrared speckle interferometry and radiative transfer modelling of the carbon star LP Andromedae

Abstract

We present the near-infrared speckle interferometry for LP And in the H and $K^{\prime}$ bands with diffraction-limited resolutions of 56 and 72 mas, new $\it JHKLM$ photometry, and the results of our radiative transfer modelling of this carbon star. The reconstructed visibility reveals a spherically-symmetric envelope surrounding the central star. To determine the physical parameters of the latter and the properties of its dusty envelope, we performed extensive radiative transfer calculations. The well-defined spectral energy distribution of LP And in the entire range from the near-IR to millimeter wavelengths (including the absorption feature visible in the stellar continuum at 3 ${\mu}$m and the shapes of the dust emission bands at 11 and 27 ${\mu}$m), together with our H -band visibility can be reproduced by a spherical dust envelope with parameters that are very similar to those of CW Leo (IRC +10 216), the best studied carbon star. For the newly estimated pulsation period $P = 617 \pm 6$ days and distance $D = 740 \pm 100$ pc, our model of LP And changes its luminosity $L_{\star}$ between 16 200 and 2900 $L_{\odot}$, its effective temperature $T_{\star}$ between 3550 and 2100 K, and its radius $R_{\star}$ between 340 and 410 $R_{\odot}$. The model estimates the star's mass-loss rate $\dot{M} \approx 1.9 \times 10^{-5}$ $M_{\odot}$ yr - 1 , assuming a constant outflow velocity $v = 14$ km s -1 . If the latter also applied to the innermost parts of the dusty envelope, then presently the star would be losing mass at a rate $\dot{M} \approx 6.0 \times 10^{-5}$ $M_{\odot}$ yr -1 . However, we believe that the inner wind velocity must actually be closer to $v \approx 4$ km s -1 instead, as wind acceleration is expected in the dust-formation zone. The dusty envelope of LP And extends from $R_1 \approx 2$ $R_{\star}$ to distances of $R_2 \approx 3$ pc from the star. The total mass of the envelope lost by the central star is $M = 3.2$ $M_{\odot}$ assuming a dust-to-gas mass ratio of $\rho_{\rm d}/\rho$ = 0.0039. The circumstellar optical depth towards the star is $\tau_{V} = 25$ in the visual. The dust model contains small silicon carbide grains, inhomogeneous grains made of a mixture of SiC and incompletely amorphous carbon, and thin mantles made of iron-magnesium sulfides. This dust mixture perfectly fits the infrared continuum and both the 11.3 ${\mu}$m and 27 ${\mu}$m emission bands. We find that our $K^{\prime}$-band visibility could not be fitted by our spherical model, so we discuss possible reasons for this interesting result. More observations are required in order to determine what causes this effect. If slight deviations from spherical geometry in its envelope are the reason, then the object's evolutionary stage would be even more similar to that of CW Leo. It appears that LP And is a highly-evolved intermediate-mass star (initial mass $M^0_{\star}$ ≈ 4 $M_{\odot}$) at the end of its AGB phase.