J/MNRAS/466/2948 Polarimetry of 53 post-AGB stars (Akras+, 2017) ================================================================================ Multi-band polarimetry of post-asymptotic giant branch stars I. Optical measurements. Akras S., Ramirez Velez J.C., Nanouris N., Ramos-larios G., Lopez J.M., Hiriart D., Panoglou D. =2017MNRAS.466.2948A (SIMBAD/NED BibCode) ================================================================================ ADC_Keywords: Stars, giant ; Polarization Keywords: polarization - circumstellar matter - binaries: general - stars: AGB and post-AGB Abstract: We present new optical broad-band (UBVRI) aperture polarimetric observations of 53 post-asymptotic giant branch (AGB) stars selected to exhibit a large near-infrared excess. 24 out of the 53 stars (45% of our sample) are presented for the first time. A statistical analysis shows four distinctive groups of polarized post-AGB stars: unpolarized or very lowly polarized (degree of polarization or DoP<1%), lowly polarized (1%8%). 23 out of the 53 (66%) belong to the first group, 10 (19%) to the second, five ( %) to the third and only three (6%) to the last group. Approximately 34%of our sample was found to be unpolarized objects, which is close to the percentage of round planetary nebulae. On average, the low and moderate groups show a wavelength-dependent polarization that increases towards shorter wavelengths, implying an intrinsic origin of the polarization, which signifies a Rayleigh-like scattering spectrum typical for non-symmetrical envelopes composed principally of small dust grains. The moderately polarized stars exhibit higher K-W3 and W1-W3 colour indices compared with the group of lowly polarized stars, suggesting a possible relation between DoP and mass-loss rate. Moreover, they are found to be systematically colder (redder in B-V), which may be associated with the condensation process close to these stars that results in a higher degree of polarization. We also provide evidence that multiple scattering in optically thin polar outflows is the mechanism that gives high DoP in post-AGB stars with bipolar or multi-polar envelopes. Description: A sample of 53 post-AGB stars was selected from De Ruyter et al. (2006A&A...448..641D) and the Torun catalogue of Galactic post-AGB and relative objects (Szczerba et al. 2007, Cat. J/A+A/469/799, 2012, Proc, IAU Symp. 283, 506) by applying the following criteria to all the programme stars: (i) they exhibit a near-IR excess in their SED profiles, which may be associated with the thermal emission of hot dust grains (Evans, 1985MNRAS.217..493L) and provides a signature of a dusty circumstellar disc/torus (De Ruyter et al., 2006A&A...448..641D) and (ii) they are bright enough at optical wavelengths to perform accurate linear polarimetry from the 0.84-m telescope at the Observatorio Astronomico Nacional in Sierra de San Pedro Martir, Mexico. In Table 1, we list for the stars of our sample their coordinates, their classification type and information about their binarity. Linear polarization measurements were performed with the 0.84-m f/15 telescope of the Observatorio Astronomico Nacional in the Sierra de San Pedro Martir, Mexico, using the POLIMA polarimeter in conjunction with a 2048x2048 CCD camera (13x13um pixels), which results in a scale of 0.44-arcsec/pix The polarimeter consists of a rotating Glan-Taylor prism driven by a stepper motor. As the polarimeter is a single-beam device with a very slow modulation, good photometric conditions are required for accurate polarimetry. The observations were performed between 2012 January and December at/or near the new Moon phase. The broad-band filters U, B, V, R and I were used with exposure times from 5 to 180s. File Summary: -------------------------------------------------------------------------------- FileName Lrecl Records Explanations -------------------------------------------------------------------------------- ReadMe 80 . This file table1.dat 93 53 Log file of the observations tableb1.dat 107 421 Intrinsic degree of polarization, PA of the polarization vectors, total degree of polarization and ISP of all the stars in our sample -------------------------------------------------------------------------------- See also: J/A+A/469/799 : Torun catalog of post-AGB and related objects (Szczerba+, 2007) Byte-by-byte Description of file: table1.dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 1- 16 A16 --- Name Name (mainly IRAS name) 19- 32 A14 --- OName Other name 35- 39 F5.1 deg GLON Galactic longitude 41- 45 F5.1 deg GLAT Galactic latitude 47- 48 I2 h RAh Right ascension (J2000) 50- 51 I2 min RAm Right ascension (J2000) 53- 56 F4.1 s RAs Right ascension (J2000) 58 A1 --- DE- Declination sign (J2000) 59- 60 I2 deg DEd Declination (J2000) 62- 63 I2 arcmin DEm Declination (J2000) 65- 68 F4.1 arcsec DEs Declination (J2000) 70- 77 A8 --- Type Type 79- 89 A11 --- Bin Note on binarity 90- 93 A4 --- Ref Reference (1) -------------------------------------------------------------------------------- Note (1): References as follows: 1 = De Ruyter et al. (2006A&A...448..641D) 2 = Van Winckel, Waelkens & Waters (1995) 3 = Hrivnak et al. (2008, Cat. J/AJ/136/1557) 4 = Hrivnak et al. (2011ApJ...734...25H) 5 = Van Winckel et al. (2012, Cat. J/A+A/542/A53) 6 = Gielen et al. (2008A&A...490..725G) 7 = Waters et al. (1993A&A...269..242W) 8 = Kreiner (2004, Cat. J/AcA/54/207) 9 = Castro-Carrizo et al. (2012A&A...545A...1C) 10 = Van Winckel et al. (1998A&A...336L..17V) 11 = Kodaira, Greenstein & Oke (1970ApJ...159..485K) 12 = Reyniers (2002, PhD thesis, Katholieke University, Leuven) -------------------------------------------------------------------------------- Byte-by-byte Description of file: tableb1.dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 1- 16 A16 --- Name Name 18- 22 F5.1 deg PAISP Intrinsic degree of polarization 24- 27 F4.1 deg e_PAISP rms uncertainty on PAISP 30- 38 F9.4 d JD Julian date (JD-2450000) 40 A1 --- Filt Filter 42- 46 F5.2 % DoP ?=- Polarization degree corrected for the ISM and bias noise 48- 51 F4.2 % e_DoP ? rms uncertainty on DoP 53- 57 F5.1 deg PA ?=- Position angle corrected for the ISM and bias noise 59- 62 F4.1 deg e_PA ? rms uncertainty on PA 64- 68 F5.2 % PMAS Observed polarization degree measurements after correction for the bias noise 70- 73 F4.2 % e_PMAS rms uncertainty on PMAS 75- 79 F5.2 % Pobs Observed polarization degree measurements before correction for the bias noise 81- 84 F4.2 % e_Pobs rms uncertainty on Pobs 86- 91 F6.2 deg PAobs Observed position angle 93- 97 F5.2 deg e_PAobs rms uncertainty on PAobs 99-102 F4.2 % ISP Interstellar degree of polarization 104-107 F4.2 % e_ISP rms uncertainty on ISP -------------------------------------------------------------------------------- History: From electronic version of the journal ================================================================================ (End) Patricia Vannier [CDS] 17-Oct-2019