Extended envelopes around Galactic Cepheids IV. T Monocerotis and X Sagittarii from mid-infrared interferometry with VLTI/MIDI

Aims. We study the close environment of nearby Cepheids using high spatial resolution observations in the mid-infrared with the VLTI/MIDI instrument, a two-beam interferometric recombiner. Methods. We obtained spectra and visibilities for the classical Cepheids X Sgr and T Mon. We fitted the MIDI measurements, supplemented by B, V, J, H, K literature photometry, with the numerical transfer code DUSTY to determine the dust shell parameters. We used a typical dust composition for circumstellar environments. Results. We detect an extended dusty environment in the spectra and visibilities for both stars, although T Mon might suffer from thermal background contamination. We attribute this to the presence of a circumstellar envelope (CSE) surrounding the Cepheids. This is optically thin for X Sgr (tau(0.55microns) = 0.008), while it appears to be thicker for T Mon (tau(0.55micron) = 0.15). They are located at about 15-20 stellar radii. Following our previous work, we derived a likely period-excess relation in the VISIR PAH1 filter, f(8.6micron)[%]= 0.81(+/-0.04)P[day]. We argue that the impact of CSEs on the mid-IR period-luminosity (P-L) relation cannot be negligible because they can bias the Cepheid brightness by up to about 30 %. For the K-band P-L relation, the CSE contribution seems to be lower (< 5 %), but the sample needs to be enlarged to firmly conclude that the impact of the CSEs is negligible in this band.

[1]  Michael W. Feast,et al.  The Luminosities and Distance Scales of Type II Cepheid and RR Lyrae variables , 2008, 0803.0466.

[2]  T. Verhoelst,et al.  The dust condensation sequence in red supergiant stars , 2009, 0901.1262.

[3]  Richard J. Mathar,et al.  MIDI the 10 m instrument on the VLTI , 2003 .

[4]  G. Fazio,et al.  GALACTIC CEPHEIDS WITH SPITZER. II. SEARCH FOR EXTENDED INFRARED EMISSION , 2010, 1011.3386.

[5]  B. Skiff,et al.  VizieR Online Data Catalog , 2009 .

[6]  Denis Gillet,et al.  High-resolution spectroscopy for Cepheids distance determination IV. Time series of Hα line profiles , 2008 .

[7]  Valery V. Kovtyukh,et al.  Phase-dependent Variation of the Fundamental Parameters of Cepheids. II. Periods Longer than 10 Days , 2005 .

[8]  H. Neilson,et al.  ON THE ENHANCEMENT OF MASS LOSS IN CEPHEIDS DUE TO RADIAL PULSATION. II. THE EFFECT OF METALLICITY , 2008, 0809.2793.

[9]  P. Kervella,et al.  Circumstellar envelopes of Cepheids: a possible bias affecting the distance scale? , 2012, Proceedings of the International Astronomical Union.

[10]  W. Freedman,et al.  Hubble Space Telescope Fine Guidance Sensor Parallaxes of Galactic Cepheid Variable Stars: Period-Luminosity Relations , 2007 .

[11]  Th. Henning,et al.  Steps toward interstellar silicate mineralogy - VII. Spectral properties and crystallization behaviour of magnesium silicates produced by the sol-gel method , 2003 .

[12]  O. Chesneau.  MIDI: Obtaining and analysing interferometric data in the mid-infrared , 2007 .

[13]  Pierre Kervella,et al.  The circumstellar envelopes of the Cepheids $\mathsf{\ell}$ Carinae and RS Puppis - Comparative study in the infrared with Spitzer, VLT/VISIR, and VLTI/MIDI , 2009, 0902.1588.

[14]  G. Bono,et al.  GALACTIC CEPHEIDS WITH SPITZER. I. LEAVITT LAW AND COLORS , 2009, 0911.2470.

[15]  Th. Henning,et al.  Aluminum Oxide and the Opacity of Oxygen-rich Circumstellar Dust in the 12-17 Micron Range , 1997 .

[16]  D. Bersier,et al.  Cepheid distances from infrared long-baseline interferometry - I. VINCI/VLTI observations of seven Galactic Cepheids , 2004 .

[17]  N. Langer,et al.  The Cepheid mass discrepancy and pulsation-driven mass loss , 2011, 1104.1638.

[18]  F. Castelli,et al.  Round Table Summary: Problems in Modelling Stellar Atmospheres , 2003 .

[19]  T. G. Barnes,et al.  Observational studies of Cepheids. II - BVRI photometry of 112 Cepheids , 1984 .

[20]  Mark R. Kidger,et al.  Spectral Irradiance Calibration in the Infrared. X. A Self-Consistent Radiometric All-Sky Network of Absolutely Calibrated Stellar Spectra , 1999 .

[21]  D. Bersier,et al.  A New Calibration Of Galactic Cepheid Period-Luminosity Relations From B To K Bands, And A Comparison To LMC Relations , 2007, 0709.3255.

[22]  G. Fazio,et al.  AN INFRARED NEBULA ASSOCIATED WITH δ CEPHEI: EVIDENCE OF MASS LOSS? , 2010, 1102.0305.

[23]  S. E. Persson,et al.  THE CARNEGIE HUBBLE PROGRAM: THE LEAVITT LAW AT 3.6 AND 4.5 μm IN THE MILKY WAY , 2012, 1209.4946.

[24]  Martin G. Cohen,et al.  THE WIDE-FIELD INFRARED SURVEY EXPLORER (WISE): MISSION DESCRIPTION AND INITIAL ON-ORBIT PERFORMANCE , 2010, 1008.0031.

[25]  R. Hindsley,et al.  The period-luminosity relation for cepheid variable stars , 1989 .

[26]  G. Perrin,et al.  Extended envelopes around Galactic Cepheids I. Carinae from near and mid-infrared interferometry with the VLTI , 2006 .

[27]  Wendy L. Freedman,et al.  CARNEGIE HUBBLE PROGRAM: A MID-INFRARED CALIBRATION OF THE HUBBLE CONSTANT , 2012, 1208.3281.

[28]  C. Alvarez,et al.  The sub-arcsecond dusty environment of Eta Carinae , 2005 .

[29]  D. Bersier,et al.  High resolution spectroscopy for Cepheids distance determination. I. Line asymmetry , 2006, 0804.1321.

[30]  S. Kanbur,et al.  Period-luminosity relations for Cepheid variables: from mid-infrared to multi-phase , 2012, 1202.0150.

[31]  Pierre Kervella,et al.  Extended envelopes around Galactic Cepheids. II. Polaris and delta Cephei from near-infrared interfe , 2006 .

[32]  Potsdam,et al.  Calibrating The Cepheid Period-Luminosity Relation From The Infrared Surface Brightness Technique I. The P-Factor, The Milky Way Relations, And A Universal K-Band Relation , 2011, 1109.2017.

[33]  K. Nordsieck,et al.  The Size distribution of interstellar grains , 1977 .

[34]  D. Majaess,et al.  ON THE FORM OF THE SPITZER LEAVITT LAW AND ITS DEPENDENCE ON METALLICITY , 2013, 1306.0011.

[35]  C. Soubiran,et al.  Reddenings of FGK supergiants and classical Cepheids from spectroscopic data , 2008, 0807.2057.

[36]  P. Kervella,et al.  Thermal infrared properties of classical and type II Cepheids - Diffraction limited 10 μm imaging with VLT/VISIR , 2011, 1111.7215.

[37]  J. Pollack,et al.  Composition and radiative properties of grains in molecular clouds and accretion disks , 1994 .

[38]  W. Gieren,et al.  Calibration of the Distance Scale from Cepheids , 2003, astro-ph/0301291.

[39]  E. al.,et al.  Extended Envelopes around Galactic Cepheids. III. Y Ophiuchi and α Persei from Near-Infrared Interferometry with CHARA/FLUOR , 2007, 0704.1825.

[40]  H. Deasy.  Observational evidence for mass loss from classical Cepheids , 1988 .