Extended Dust Emission from Nearby Evolved Stars★

Abstract

We present JCMT SCUBA-2 450µm and 850µm observations of 14 Asymptotic Giant Branch (AGB) stars (9 O-rich, 4 C-rich and 1 S-type) and one Red Supergiant (RSG) in the Solar Neighbourhood. We combine these observations with Herschel /PACS observations at 70µm and 160µm and obtain azimuthally-averaged surface-brightness profiles and their PSF subtracted residuals. The extent of the SCUBA-2 850 µm emission ranges from 0.01 to 0.16 pc with an average of ∼ 40% of the total flux being emitted from the extended component. By fitting a modified black-body to the four-point SED at each point along the radial profile we derive the temperature (T ), spectral index of dust emissivity (β) and dust column density (Σ) as a function of radius. For all the sources, the density profile deviates significantly from what is expected for a constant mass-loss rate, showing that all the sources have undergone variations in mass-loss during this evolutionary phase. In combination with results from CO line emission, we determined the dust-to-gas mass ratio for all the sources in our sample. We find that, when sources are grouped according to their chemistry, the resulting average dust-to-gas ratios are consistent with the respective canonical values. However we see a range of values with significant scatter which indicate the importance of including spatial information when deriving these numbers.

The sub-mm variability of IRC+10216 and o Ceti

“…Table 1 presents the average flux, standard deviation and the flux range derived from PSF photometry for each source at each wavelength. The average fluxes of both sources at both wavelengths are consistent with the respective total fluxes reported in Dharmawardena et al (2018).…”

“…Optical emission would peak at the moment of the stellar pulse, when the star is at its brightest. The corresponding travel time is ∼ 50 days (0.04 pc; Dharmawardena et al 2018), too short to explain the phase lag. Groenewegen et al (2012) identify a lag of 402±37 days between the bow shock and the central source of IRC+10216 in the far-IR, which they attribute to light travel time between the star and the bow shock.…”

“…Instead we present snapshots built along the bolomet- ric luminosity light curve of IRC+10216 which allows us to generate a snapshot light curve in the sub-mm ignoring the time dependent effects. The stellar parameters were taken from Menten et al (2006Menten et al ( , 2012 and the dust parameters are derived by Dharmawardena et al (2018). Key stellar parameters used are as follows: (i) Maximum stellar luminosity (Lmax): 1.1 × 10 4 L ; (ii) Minimum stellar luminosity (Lmin): 4400 L ; (iii) Effective stellar temperature: 2750 K; (iv) Stellar radius (R * ): 1.9 AU.…”

“…The thick dusty envelope also makes it one of the brightest near-IR sources in the sky. We have measured the envelope which extends out to ∼ 1 in the sub-mm continuum at 450µm and 850µm (Dharmawardena et al 2018) using the James Clerk Maxwell Telescope's Sub-millimetre Common Use Bolometer Array-2 instrument (SCUBA-2; Holland et al 2013).…”

The nearby evolved stars survey – I. JCMT/SCUBA-2 submillimetre detection of the detached shell of U Antliae

“…These SEDs are fitted with a single-temperature blackbody model modified by an effective emissivity law (e.g., Hildebrand 1983; Gordon et al 2014) using the method presented in Sec. 3.2 of Dharmawardena et al (2018). This results in radial profiles for the dust T , β and Σ.…”