Science

Vega Phenomenon
ISOPHOT's view on the Vega-phenomenon
P. Ábrahám, A. Moór, Cs. Kiss

 

The presence of circumstellar dust around main-sequence stars ("Vega phenomenon") was the topic of several ISOPHOT key projects. Observations belonging to the different programmes, however, were reduced in different ways, and the results of the projects were published separately. The only compilation of an ISOPHOT Vega-star list (Decin et al. 2003) was based on the original papers, and inherited the inhomogeneous quality of the data processing. In our study we systematically re-analyse the ISOPHOT observations of normal stars, and present the final ISOPHOT catalogue of Vega-candidate stars.

Motivation
Vega-phenomenon was one of the main discoveries of the IRAS mission. However, the spatial resolution and the sensitivity of the IRAS detectors imposed limitations on detailed studies of the Vega candidate systems. Higher angular resolution coronagraphic observations at optical wavelengths indicated that in a number of cases the far-infrared excess observed by IRAS is of interstellar, rather than of circumstellar origin, leading to false entries in the Vega-candidate lists (e.g. Kalas et al. 2002). For sensitivity reasons several important questions could not be properly addressed from the IRAS data, e.g.: "Does the presence of a disk depend on the stellar age?" or "Is the incidence of disks the same in clusters and multiple systems as among single field stars?".

Taking advantage of the higher spatial resolution and sensitivity of ISOPHOT, a number of key observing programmes were devoted to the Vega phenomenon. The aims of these programmes were to obtain multiwavelength photometry and resolved maps of the most famous Vega-type stars at far-infrared wavelengths, and also to address the temporal evolution of the disks. The observations belonging to the different projects, however, were reduced in different ways, and the results of the projects were published in separate papers. The only compilation of an ISOPHOT Vega-star list, presented by Decin et al. (2003), was based on the original papers, and inherited the inhomogeneous quality of the processed data.

In the present study we systematically check the list of ISOPHOT observations of normal stars (the observations have already been re-processed in a homogeneous way in the framework of our earlier calibration projects), and compile a catalogue of Vega-candidate stars by applying identical detection criteria to all stars from all programmes. Such a study could give an independent confirmation of the number of excesses found in the original papers, and the resulting list would be an input for the next infrared space missions.

Data reduction
Most ISOPHOT Vega programmes used the PHT22 mini-map observing mode, thus in the following we restrict our investigation on observations obtained in this mode only. After carefully checking the ISOPHOT archive, we selected 354 far-infrared mini-map observations of 198 normal stars, and re-processed them according to our latest knowledge on the ISOPHOT C100 and C200 detector calibration. Details on the selection and data reduction processes are given in our calibration report, and the final flux values are listed here.

Selection of Vega-candidate stars
Following the principles of the method by Plets & Vynckier (1999), for each selected star we predicted the far-infrared flux density of the stellar photosphere using the Ks-band magnitude (or V-band, when good quality Ks photometry was not available) and the B-V color index. Ks-band photometry was drawn from the Two Micron All Sky Survey (2MASS) catalog (Cutri et al. 2003), V magnitudes and B-V color indices were taken from the Hipparcos and Tycho Catalogues. As a first step a photospheric 25 micron flux density was derived from the Ks magnitude and the B-V color of the star using the collection of stellar model predictions by M.Cohen and P.Hammersley (available on the ISO Data Centre home page). Then, color relationships predicting the photospheric flux ratios between 25 micron and the ISO photometric bands were also derived from the same stellar models. The average accuracy of the predicted far-infrared fluxes is estimated to be around 4% when computed from the Ks-magnitudes, and 8% when computed from V-magnitudes. In order to compute IR excess values the predicted photospheric flux densities were subtracted from the measured flux densities in each ISO band. In principle the ISO fluxes have to be color corrected since the shape of the spectral energy distribution of the system usually differs from the Fν ∼ ν-1 reference spectrum (this spectral shape was assumed while the flux densities quoted in the ISO catalogues were derived from the detector in-band powers). Since the true spectrum of the system is not known a priori, we decided to multiply the predicted photospheric fluxes - rather than dividing the ISO flux densities - with color correction factors appropriate for a stellar photosphere (ISO Handbook Vol. IV, Laureijs et al., 2003). The significance level of the infrared excess was calculated in each photometric band with the following formula:



where δFmeas is the quoted uncertainty in the FSC or SSC and δFpred is the uncertainty of the prediction described above. When Sexcess was greater than 3 either in the 25 or 60 μm bands, the object was selected as an excess candidate star. Applying the above criteria we identified in total 43 excess candidate stars in the ISO database.


Computing fractional luminosity values
In order to compute fractional luminosity value for each candidate star, we constructed spectral energy distributions by combining infrared fluxes from ISOPHOT and from IRAS and Spitzer when available. The excess above the predicted photosphere was fitted by a single temperature modified blackbody, where the emissivity was assumed to vary as 1 - exp[-(λ0/λ)β], where λ0 was set to 100 μm (see, e.g., Williams et al. 2004). We fixed β equal to 1, which is a typical value in the case of debris systems (Dent et al. 2000). If the excess was detected at one wavelength only, we adopted a modified blackbody whose peak (in Fν) coincided with that single wavelength. From the fitted spectral shape color correction factors were computed and applied to the data. Then again a modified blackbody was fitted resulting in new color correction factors, and this procedure was repeated until the color correction factors converged. Finally, the fractional dust luminosity was calculated as fd = LIR/Lbol. In order to estimate the uncertainties on our fractional luminosity values we performed a Monte Carlo simulation. We added Gaussian noise to the photometric data points using their quoted photometric errors and then recomputed the fractional dust luminosities. Formal uncertainties of the predicted theoretical photospheric fluxes were also taken into account. Final uncertainties were derived as the standard deviation of these values after 1000 repetitions. We note that these values include only random uncertainties; systematic errors due to e.g. limited wavelength coverage are not taken into account.

List of Vega-candidate stars detected by ISOPHOT.
Below we list those 43 Vega-candidate stars where the ISOPHOT photometry exhibits far-infrared excess.

Name Other name V B-V Distance Spectral type $f_d$ $\sigma (f_d)$
[mag] [mag] [pc] $[10^{-4}]$ $[10^{-4}]$
HD 105 7.51 0.595 40 G0V 2.50 0.30
HD 8907 6.66 0.505 34 F8V 2.40 0.10
HD 9672 49Cet 5.62 0.066 61 A1V 9.20 0.60
HD 10647 5.52 0.551 17 F8V 3.00 0.30
HD 10700 $\rm\tau\,Cet$ 3.49 0.727 3.6 G8V 0.08 0.02
HD 15115 6.79 0.399 44 F2 4.90 0.40
HD 15745 7.47 0.360 64 F0 20.10 1.40
HD 17390 6.48 0.387 45 F3IV/V 1.90 0.20
HD 17925 6.04 0.862 10 K1V 0.32 0.07
HD 18907 $\rm\epsilon\,For$ 5.87 0.794 30 G5IV 0.22 0.06
HD 20794 82Eri 4.26 0.711 6.1 G8V 0.08 0.02
HD 22049 $\rm\epsilon\,Eri$ 3.72 0.881 3.2 K2V 0.78 0.07
HD 22484 10Eri 4.29 0.575 14 F9V 0.09 0.02
HD 25457 5.38 0.516 19 F5V 1.00 0.20
HD 30447 7.85 0.393 78 F3V 7.50 1.10
HD 30495 58Eri 5.48 0.632 13 G3V 0.43 0.06
HD 33262 $\rm\zeta\,Dor$ 4.71 0.526 12 F7V 0.11 0.02
HD 35850 6.30 0.553 27 F7V 0.28 0.07
HD 37484 7.26 0.404 60 F3V 2.70 0.50
HD 38207 8.47 0.391 100 F2V 10.80 0.60
HD 38678 $\rm\zeta\,Lep$ 3.55 0.104 22 A2Vnn 1.10 0.20
HD 39060 $\rm\beta\,Pic$ 3.85 0.171 19 A3V 24.30 1.10
HD 53143 6.81 0.786 18 K0IV-V 2.00 0.50
HD 72905 58UMa 5.63 0.618 14 G1V 0.30 0.02
HD 95418 $\rm\beta\,UMa$ 2.34 0.033 24 A1V 0.05 0.01
HD 110058 7.99 0.148 100 A0V 18.90 3.30
HD 128167 $\rm\sigma\,Boo$ 4.47 0.364 15 F3Vwvar 0.06 0.02
HD 139664 gLup 4.64 0.413 18 F5IV-V 0.77 0.04
HD 164249 7.01 0.458 47 F5V 10.40 1.60
HD 170773 6.22 0.429 36 F5V 3.80 0.40
HD 172167 Vega 0.03 -0.001 7.8 A0V 0.21 0.07
HD 181296 $\rm\eta\,Tel$ 5.03 0.020 48 A0Vn 2.40 0.20
HD 181327 7.04 0.480 51 F5/F6V 29.30 1.60
HD 191089 7.18 0.480 54 F5V 19.10 2.20
HD 192758 7.02 0.317 67 F0V 5.60 0.50
HD 202917 8.65 0.690 46 G5V 2.90 0.80
HD 206893 6.69 0.439 39 F5V 2.30 0.20
HD 207129 5.57 0.601 16 G2V 0.99 0.15
HD 209253 6.63 0.504 30 F6/F7V 0.77 0.09
HD 213617 6.43 0.350 53 F1V 0.75 0.07
HD 216956 Fomalhaut 1.17 0.145 7.7 A3V 0.80 0.06
HD 218396 5.97 0.259 40 A5V 2.30 0.20
HD 221853 7.35 0.405 71 F0 8.00 1.10

Related papers:

Title: Circumstellar dust around main-sequence stars: what can we learn from the ISOPHOT archive?
Authors: Ábrahám, Péter; Moór, Attila; Kiss, Csaba; Héraudeau, Philippe; del Burgo, Carlos
Journal: Exploiting the ISO Data Archive. Infrared Astronomy in the Internet Age, held in Siguenza, Spain 24-27 June, 2002. Edited by C. Gry, S. Peschke, J. Matagne, P. Garcia-Lario, R. Lorente, & A. Salama. Published as ESA Publications Series, ESA SP-511. European Space Agency, 2003, p. 129.

Title: Nearby Debris Disk Systems with High Fractional Luminosity Reconsidered
Authors: Moór, Attila; Ábrahám, Péter; Derekas, Alíz; Kiss, Csaba; Kiss, L. László; Apai, Dániel; Grady, Carol; Henning, Thomas
Journal: The Astrophysical Journal, Volume 644, pp. 525-542, 2006