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A Common Proper Motion Companion to the Exoplanet Host 51 Pegasi 4 John Greaves

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A Common Proper Motion Companion to the Exoplanet Host 51 Pegasi 4 John Greaves University of South Alabama Journal of Double Star Observations VOLUME 2 NUMBER 1 WINTER 2006 We have lots of great articles in this issue including new measurements from the Dave Arnold and an article from Brian Mason describing data files from the USNO. We also have an article about the Aitken criterion from Francisco Rica Romero. Robert G. Aitken (1864—1951) was the grand old man of double star astronomy and still observed doubles visually long http://www.phys-astro.sonoma.edu after others had turned to photography. He ranks sixth on the list of double star observations, was on the board of directors of the Astronomical Society of the Pacific for decades, author of the wonderful book The Binary Stars, and was hearing impaired. Check out Rica Romero's article on page 36. NEW WEB ADDRESS The JDSO now has a new web address. You must have learned that by now or you wouldn't be reading this! Anyway, we knew that our old url was very awkward and inappropriate, so we acquired www.jdso.org. If you forget and go to our old address by mistake, that's OK, you will be transferred to the new address automatically. Robert Grant Aitken Thanks for looking us up! Inside this issue: Is HLD 32 A (= WDS 18028-2705 A) an Unresolved Binary Candidate? 2 Francisco M. Rica Romero A Common Proper Motion Companion to the Exoplanet Host 51 Pegasi 4 John Greaves Divinus Lux Observatory Bulletin: Report #1 6 Dave Arnold Divinus Lux Observatory Bulletin: Report #2 13 Dave Arnold Requested Double Star Data from the U.S. Naval Observatory 21 Brian Mason R.G. Aitken's Criterion to Detect Physical Pairs 36 Francisco Rica Romero Vol. 2 No. 1 Winter 2006 Journal of Double Star Observations Page 2 Is HLD 32 A (= WDS 18028-2705 A) an unresolved binary candidate? Francisco Rica Romero Astronomical Society of Mérida (Spain), LIADA’s Double Star Section (Argentina) [email protected] Abstract: The double star HLD 32 = WDS18028-2705 was studied by the LIADA group in 2002 using BVJHK photometry, astrometric measures, and proper motions from the litera- ture. According to this study, HLD 32 is a physical pair. Their members have very similar spectral distribution (F6V and F7V) but a flux ratio of about +0.85 magnitudes. One strong explanation is the possible binary nature of primary component. ference could be caused by errors in observational HLD 32 is a system located at 18h 02m 45s and data or in our study. But there is another, more inter- –27º 04' 42" in the constellation Sagittarius composed esting explanation. If the primary were composed of of components with magnitudes 8.37 and 9.2 and two components, then its distance modulus would be separated by 5.0". This system was first measured in up to 0.75 magnitude smaller than that for the 1881 by S.W. Burnham. It has been measured 15 bounded secondary. So, is the primary component an times. Figure 1 is a DSS image of this system. unresolved pair? This is very probable, since last Relative motion was calculated using historical studies indicate that more than 50% of stars belong to measures from the WDS in addition to LIADA’s meas- multiple stellar systems. ures (Rica Romero, 2005a), correcting theta values for precession and proper motions. The results were Δμ(α) = +3” yr-1 and Δ μ (δ) = -2” yr-1. Spectral types and luminosity class were obtained using BVJHK optical-infrared photometry and proper motions from Tycho-2 and 2MASS catalogs (Rica Ro- mero, 2005b). The spectral types were corrected by reddening using Schlegel et al. (1999) and F. Paresce (1984) galactic interstellar maps. The cosecant law was used to correct the galactic extinction for dis- tance. HLD 32 is located at galactic longitude of 3.5º and galactic latitude of –2.3º, so it is in the galactic plane. A value of E(B-V) = 0.07 was obtained. Photo- metric distances were corrected by reddening. Astro- physical data for HLD 32 are given in Tables 1 and 2. Several professional methods were used to con- clude that HLD 32 is a binary system and so a physi- cal pair (Rica Romero, 2005a). HLD 32 is composed by stars with F6V and F7V spectral types with Δ(V-Mv)= +0.63. The flux ratios were ΔV= +0.83, ΔJ= +0.89, ΔH=+0.80, and ΔK= +0.88, indicating that the spectral distribution is nearly the same for both components. Since HLD 32 is a physical pair then both compo- Figure 1: Photographic plate of HLD 32 from Digitized Sky Sur- nents must be located at the same distance, but the vey II taken in 1991.682. HLD 32 is located in a crowded region of the Milky Way in Sagittarius. distance modulus differs by 0.63 magnitude. This dif- Vol. 2 No. 1 Winter 2006 Journal of Double Star Observations Page 3 Is HLD 32 A (= WDS 18028-2705 A) an unresolved binary candidate? Those interested in this study can request from Bibliography me more detailed information about this investiga- Rica Romero, F. M., 2005b, JDSO, 1, 8. tion. In order to confirm the possible binary nature of Rica Romero, F. M., 2005a, JDSO, 1, 24. HLD 32 A, the help of amateurs and professionals is D.J. Schlegel, D.P. Finkbeiner, & M. Davis, 1998, needed. Those interested in it, could try to split HLD "Maps of Dust Infrared Emission for Use in Esti- 32 A using telescopes with big apertures. Speckle in- mation of Reddening and Cosmic Microwave Back- terferometery and the study of radial velocity could be ground Radiation Foregrounds", ApJ, 500, 525 the best way to confirm the nature of this star. So, if there is any professional interested in this project, Burstein, D.; Heiles, C., 1984, "Reddening estimates please feel free to include this star in your observa- for galaxies in the Second Reference Catalog and tional program. I would like to know the positive or the Uppsala General Catalog", ApJS, 54, 33 negative results obtained. You can contact me at fri- F. Paresce, 1984, "On the distribution of interstellar [email protected]. matter around the sun", AJ, 89, 1022 Primary Secondary V 8.37 a) 9.2 d) B-V a) 0.55 K b) 6.91 7.79 J-H b) 0.26 0.35 H-K b) 0.06 -0.02 Mv c) +3.6 +3.8 V-Mv c) 4.56 5.19 (μ(α),μ(δ))a (-0.060 , +0.010 ) ”/yr Sp c) F6V f) F7V Table 1. WDS 18028-2705 = HLD 32 AB. Astrophysical data Note: a) Tycho-2 catalogue; b) 2MASS catalogue; c) This work; spectral type and distance modulus are corrected by redden- ing; d) Inferred by WDS historical photometry; f) In literature A is F7V RA: 18h 02m 45s0 DEC: –27º 04 42" (l , b) galatic (+3.5º , -2.3º) coordinate: (θ / ρ, epoch): 101.2º / 5.05", 1998.212 (Δμ(α), Δμ(δ)): (+0.003 , -0.002) "/yr Δ(V-Mv): +0.63 E(B-V): +0.07 Distance: 95 pc Table 2: Data for binary WDS 18028-2705 = HLD 32 AB Vol. 2 No. 1 Winter 2006 Journal of Double Star Observations Page 4 A Common Proper Motion Companion to the Exoplanet Host 51 Pegasi John Greaves Northampton, U.K. Abstract: The exoplanet 51 Pegasi has a widely separated red dwarf companion. Two other, far more distant, stars are also co-moving with this star, showing that they are at least of common Galactic orbit due to a common origin. companion of 1.2 Mo or thereabouts. Such an orbital Introduction period is unlikely, and the pair are probably not gravi- The somewhat Sun-like nearby G dwarf star 51 tationally connected. Pegasi was shown to be host to a substellar compan- That the companion is a red dwarf is shown by ion, or "exoplanet", ten years ago (Mayor and Queloz combining the V 15.61 given by Salim and Gould 1995), the exoplanet being estimated as a roughly half (2004) with the 2MASS J magnitude of 11.96 and Ks Jovian mass object orbiting the G star roughly every magnitude of 11.11 (Cutri et al 2003) to give J-Ks of 4.2 days at a distance of 0.05 Astronomical Units 0.85 plus V-Ks of 4.50 (taking the reasonable assump- (AU). tion of negligible difference between 2MASS Ks and Proper motion data shows that this high proper various other K bands, see Cutri et al 2003 again) and motion star is not moving alone in space, with at least using Figure 2 of Bessell and Brett (1988) to see that one if not three distant co-moving companions. these values lie well along and within the branch of The Nearer Comoving Star objects that are defined within that article's text as delineating the red dwarfs. The Hipparcos Catalog (ESA 1997) shows that the This V magnitude of 15.61 equates to an absolute proper motion of 51 Pegasi is 208 milliarcseconds per magnitude, MV, of 14.7, taking the viewpoint that as year (masy-1) in Right Ascension (RA), with the correc- this star is truly a companion to 51 Pegasi which tion made for the cosine of the Declination, and 61 means it should be more or less at the same distance masy-1 in Declination (Dec) with errors <1 masy-1. of 15.3 parsecs, and this absolute magnitude value is The UCAC2 (Zacharias et al 2004) reveals a star not inappropriate for a cooler spectral class M dwarf.
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