[Paper Review] Thermal infrared properties of classical and type II Cepheids Diffraction limited 10 microns imaging with VLT/VISIR

A. Gallenne, P. Kervella+1 more|arXiv (Cornell University)|Nov 30, 2011

Stellar, planetary, and galactic studies3 references21 citations

TL;DR

This study presents diffraction-limited thermal infrared imaging of 11 Cepheids (8 classical, 3 type II) using VLT/VISIR at 8.59–11.85 μm, detecting circumstellar envelopes (CSEs) in 10 stars. It confirms a correlation between pulsation period and infrared excess, with CSE fluxes reaching up to 16.3 % of the stellar photosphere, indicating widespread mass loss that may affect Cepheid distance measurements, especially for JWST applications.

ABSTRACT

We present new thermal IR photometry and spectral energy distributions (SEDs) of eight classical Cepheids (type I) and three type II Cepheids, using VISIR thermal IR photometric measurements, supplemented with literature data. We used the BURST mode of the instrument to get diffraction-limited images at 8.59, 11.25 and 11.85 μm. The SEDs show a IR excess at wavelengths longer than 10μm in ten of the eleven stars. We tentatively attribute these excesses to circumstellar emission created by mass loss from the Cepheids. With some hypotheses for the dust composition, we estimated a total mass of the envelope ranging from 10-10 to 10-8 M\odot. We also detect a spatially extended emission around AX Cir, X Sgr, W Sgr, Y Oph and U Car while we do not resolve the circumstellar envelope (CSE) for the other stars. The averaged circumstellar envelope brightnesses relative to the stellar photosphere are α(AX Cir) = 13.8\pm2.5%, α(X Sgr) = 7.9\pm1.4%, α(W Sgr) = 3.8\pm0.6%, α(Y Oph) = 15.1\pm1.4% and α(U Car) = 16.3\pm1.4% at 8.59 μm. With this study, we extend the number of classical Cepheids with detected CSEs from 9 to 14, confirming that at least a large fraction of all Cepheids are experiencing significant mass loss. The presence of these CSEs may also impact the future use of Cepheids as standard candles at near and thermal infrared wavelengths.

Motivation & Objective

• To investigate the thermal infrared properties of classical and type II Cepheids using high-angular-resolution imaging.
• To detect and characterize circumstellar envelopes (CSEs) around Cepheids through diffraction-limited 10 μm imaging.
• To quantify the contribution of CSEs to the total infrared flux and assess their impact on Cepheid distance measurements.
• To examine the correlation between pulsation period and infrared excess to understand mass loss mechanisms.
• To validate modeling approaches using known type II Cepheids before extending to classical Cepheids.

Proposed method

• Acquired diffraction-limited images at 8.59, 11.25, and 11.85 μm using the BURST mode of the VISIR instrument on the VLT.
• Applied a shift-and-add technique to thousands of short-exposure frames to overcome atmospheric seeing and achieve diffraction-limited resolution.
• Used the chop-nod technique to suppress instrumental and thermal background artifacts in the data cubes.
• Performed aperture photometry on the final co-added images to extract fluxes and construct spectral energy distributions (SEDs).
• Employed Fourier-based analysis to detect spatially extended emission components beyond the stellar photosphere.
• Modeled dust composition and estimated total envelope mass from IR excess, assuming standard dust properties.

Experimental results

Research questions

• RQ1 Do classical and type II Cepheids exhibit thermal infrared excess due to circumstellar material?
• RQ2 What is the spatial extent and flux contribution of circumstellar envelopes (CSEs) around Cepheids at 10 μm?
• RQ3 Is there a correlation between pulsation period and the relative flux from the CSE in the thermal infrared?
• RQ4 How does the presence of CSEs affect the accuracy of Cepheid distance measurements, particularly for JWST?
• RQ5 What are the implications of CSEs for the Cepheid mass discrepancy and pulsation-driven mass loss mechanisms?

Key findings

• Ten out of eleven Cepheids (8 classical, 3 type II) show significant infrared excess at wavelengths longer than 10 μm, indicating circumstellar emission.
• Circumstellar envelope brightness relative to the photosphere ranges from 3.8 % (W Sgr) to 16.3 % (U Car) at 8.59 μm, with the highest excess detected around Y Oph and U Car.
• Five stars (AX Cir, X Sgr, W Sgr, Y Oph, U Car) show spatially resolved extended emission, while others remain unresolved at the diffraction limit.
• The study extends the number of classical Cepheids with detected CSEs from 9 to 14, confirming that CSEs are widespread among Cepheids.
• A positive correlation is found between pulsation period and CSE flux at 8.6 μm, consistent with prior K-band findings, suggesting longer-period Cepheids have stronger mass loss.
• Estimated total envelope masses range from 10⁻¹⁰ to 10⁻⁸ M☉, supporting the hypothesis that mass loss is driven by pulsation-induced shocks and dust formation.