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A&A
Volume 587, March 2016
Article Number A51
Number of page(s) 8
Section Catalogs and data
DOI https://doi.org/10.1051/0004-6361/201527965
Published online 16 February 2016

© ESO, 2016

1. Introduction

Stellar binaries and multiple systems appear to be the main product of star formation if the primaries of these systems are at least as massive as the Sun. The solar neighbourhood, representing the average Galactic disk population of stellar systems that typically have existed for several Gyrs, is naturally one of the best-investigated regions with respect to multiplicity. The Research Consortium on Nearby Stars (RECONS)1 states a multiplicity rate, i.e. the probability that a given system has more than one component, of 29% for their 10 pc sample. However, this overall “low rate is because M-type dwarfs dominate the solar neighbourhood (a full 73% of the stellar sample ... are M-type dwarfs), and do not have companions as often as their more massive stellar cousins”. Among the 100 nearest RECONS systems (with a horizon of about 6.5 pc), there are 70 where M dwarfs are the most-massive component, of which only 18 (26%) are known to have companions. On the other hand, there are 22 AFGK primaries, of which 12 (55%) are in known multiple systems. The RECONS 10 pc census shows a strong increase from 2000 to 2012 both in the number of M dwarfs (+25%) and in the number of stellar and LT-type companions (+26%).

The multiplicity of F- and G-type stars in a wider solar neighbourhood was the focus of investigations by Fuhrmann & Chini (2012, 2015), Chini et al. (2014), Tokovinin (2011, 2014), and Tokovinin & Lépine (2012). To distinguish between visual and physical wide double stars, the common proper motion (CPM) of the components is often used as a criterion. In the era of photographic sky surveys, Luyten (1997) compiled a catalogue of CPM pairs. Frankowski et al. (2007) investigated CPM binaries in the Hipparcos catalogue, where both components are Hipparcos stars, compared their short-term (Hipparcos) and long-term (Tycho-2) proper motions, and used radial velocities as control data. The systematic search for faint CPM companions to Hipparcos stars (Gould & Chanamé 2004; Lépine & Bongiorno 2007) not only improved the statistics of the multiplicity of AFGK stars, but also allowed for a better characterisation of the lower-mass companions by making use of the knowledge about the primaries. One of the physical parameters of interest in that respect is the metallicity of M dwarfs and subdwarfs (e.g. Li et al. 2014). The CPM method continues to be useful, in particular for new deep surveys (e.g. Ivanov et al. 2013; Deacon et al. 2014). Focusing on a sample of nearby F stars, we demonstrate how much we can still improve the CPM statistics and find previously overlooked wide companions, even of well-known bright stars, in existing public surveys. The main results of this research note, which includes data on new, confirmed, and rejected CPM companions, are listed in Tables A.1, B.1, and C.1, respectively.

2. Sample definition and search method

2.1. SIMBAD F stars sample

From 1188 stars, for which SIMBAD lists spectral types F0 to F9 and parallaxes larger than 20 mas, we selected 545 with proper motions larger than 150 mas/yr to search for CPM companions. Our high proper motion 50 pc sample overlaps in part with the sample of Tokovinin (2014), which is about nine times larger. His sample reaches out to 67 pc, excludes stars with large parallax errors (>7 mas), and contains both F and G stars, but with a colour selection only corresponding to late-F and early-G stars (approximately F5V to G6V). Finally, he included only dwarfs and subgiants. We are aware of possible uncertainties (e.g. missing updates) in the SIMBAD data, in particular the spectral types (see also Scholz et al. 2015 concerning F-type subdwarfs).

2.2. Visual inspection and catalogue search

In our visual inspection of the sky areas around 545 F stars, we used IRSA finder charts tools2 in several runs with image sizes from 0.5 arcmin to 3 arcmin. In addition, we extracted 1 arcmin and 2 arcmin finder charts from the UKIRT deep infrared sky survey (UKIDSS) and the visible and infrared survey telescope for astronomy (VISTA) archives3. We considered the Two Micron All-Sky Survey (2MASS; Skrutskie et al. 2006) images with their high resolution and dynamic range as reference and searched for CPM candidates in other images with epoch differences large enough to show a change in position similar to that of the primaries. Special attention was paid to objects overlapping with diffraction spikes and other known artifacts, as well as to those with very red or blue colours expected for very low-mass and white dwarf (WD) companions, respectively. A few cases are illustrated by selected finder charts in Figs. D.1−D.3.

Our search was sensitive to angular separations from a few arcseconds to a few arcminutes. It was complementary to the work of Tokovinin & Lépine (2012) that aimed at angular separations of the companion larger than 30 arcsec. Using existing sky surveys, we tried to find CPM companions that have small separations. However, to detect very close companions with angular separations of the order of 1 arcsec or less, dedicated high-resolution imaging observations (e.g. Ehrenreich et al. 2010; Meshkat et al. 2015) are required.

Using the CDS cross-match service4 we also checked the fourth US Naval Observatory CCD Astrograph Catalogue (UCAC4; Zacharias et al. 2013) and the first US Naval Observatory Robotic Astrometric Telescope catalogue (URAT1; Zacharias et al. 2015) for possible new CPM companions within 2 arcmin of the primaries. We note that these catalogues were already the subject of CPM searches by Hartkopf et al. (2013) and Nicholson (2015). In our catalogue search, we did not require a correct proper motion measurement of the bright and sometimes problematic primaries in the given catalogue.

2.3. Proper motion measurements and CPM status

In our proper motion determinations we combined multi-epoch positional measurements from the following surveys (roughly sorted by epochs) if available:

  • 1)

    photographic Schmidt plates scanned with the Automated Photographic Measuring (APM; McMahon et al. 2000) and SuperCOSMOS Sky Surveys (SSS; Hambly et al. 2001);

  • 2)

    2MASS (Skrutskie et al. 2006);

  • 3)

    DEep Near-Infrared southern sky Survey (DENIS; Epchtein et al. 1997);

  • 4)

    UCAC4 (Zacharias et al. 2013);

  • 5)

    Carlsberg Meridian Catalogue (CMC; Muiños & Evans 2014);

  • 6)

    Sloan Digital Sky Survey (SDSS; Abazajian et al. 2009);

  • 7)

    INT PHotometric Hα Survey (IPHAS; Barentsen et al. 2014);

  • 8)

    UKIDSS Large Area (UKIDSS LAS; Lawrence et al. 2007) and Galactic Plane Surveys (UKIDSS GPS; Lucas et al. 2008);

  • 9)

    Wide-field Infrared Survey Explorer (WISE; Wright et al. 2010);

  • 10)

    URAT1 (Zacharias et al. 2015);

  • 11)

    VISTA Hemisphere (VHS; McMahon et al. 2013) and Variables in the Via Lactea Surveys (VVV; Minniti et al. 2010).

Those CPM candidates with small separations that were not well-measured or were absent in the SSS/2MASS/DENIS/SDSS catalogues were detected visually in the corresponding FITS images; we performed this using the ESO skycat tool. Depending on the number of epochs available, the accuracy of the simple linear proper motion fit that used all input positions with equal weights varied considerably. No attempt was made to transform the target positions in different surveys to a common system before the proper motion fit as we expected individual centroiding errors affected by the close bright primaries to be larger (\hbox{$\gtrapprox$}100 mas) than systematic errors.

The proper motion errors of the CPM companions were typically much larger than those of the known primaries, although in some cases we achieved a high precision for the CPM companion and excellent agreement with the known proper motion of its primary. For the majority of the 19 new and 31 confirmed CPM companions shown in Tables A.1 and B.1, respectively, their proper motion components agreed to within 2σ of the formal errors with those of the primaries. For the primaries, we preferred the longer-time baseline proper motions of the UrHip (Frouard et al. 2015) or Tycho-2 (Høg et al. 2000) catalogues instead of the Hipparcos (van Leeuwen 2007) values. A few CPM companions, for which at least one of the proper motion components agreed only to within 3σ with that of the primary, are considered CPM candidates and are flagged with (?), whereas two objects with even larger discrepancies are considered doubtful CPM objects and are flagged with (??). Our overall level of agreement of the proper motion components is similar to that of Deacon et al. (2014), who described 57 new faint CPM objects of Hipparcos stars with total proper motion differences of less than 5σ (their Eq. (1)).

2.4. Spectral type estimates

The spectral types of red CPM companions (according to their near-infrared 2MASS, UKIDSS, or VISTA colours, and DENIS I−J colours if available) were estimated based on the known distances of the primaries and the relation between absolute J magnitude and (early-K to late-L) spectral type from Scholz, Meusinger & Jahreiß (2005). Our spectral type estimates are in good agreement (within 0.5 subtypes) with previous classifications available for six M and two early-L dwarfs (Tables A.1 and B.1). Blue objects, simply assumed to be WD candidates, as well as possible subdwarf candidates are discussed in Sect. 4.

3. Results

The results of our survey are presented in Tables A.1, B.1, and C.1, listing new, confirmed, and rejected CPM companions together with their F star primaries, respectively. All 19 new CPM objects belong to primaries with distances between 25 and 50 pc according to their Hipparcos parallaxes. Among the 31 confirmed CPM companions, there are four within 25 pc (with parallaxes of 40−75 mas). One new object (HD 76493B) is a doubtful CPM companion not only because of the poor agreement of μαcosδ, but also because of the discrepancy between the short- and long-term proper motions of its primary HD 76493A. Such discrepancies hinting at the influence of unresolved companions are also seen for the known close binary HD 2057AB and for several other primaries in both Tables A.1 and B.1. If confirmed as CPM, HD 76493B would have the largest projected physical separation of all our targets (≈2600 AU).

4. Brief discussion and notes on individual objects

In addition to the objects listed in Tables A.1 and B.1 there are 98 other known CPM objects among the 545 F stars (18%), mainly at small (<5 arcsec) or large (>120 arcsec) angular separations and of earlier spectral types (FGK). With our 19+31 new and confirmed CPM companions we add a further 9% (50% more CPM objects), which are mainly M dwarfs at intermediate angular separations. On average, both new and confirmed objects have projected physical separations of about 900 AU (ranging from about 200 AU to 2500 AU). Certainly, the high-resolution astrometric measurements of Gaia will bring the multiplicity rate of the F stars in our high proper motion 50 pc sample to the 50% level, as currently known for the small RECONS 6.5 pc sample, or even higher. Gaia will also provide accurate distances for individual system components and the CPM status for all nearby objects including those with relatively small proper motions.

We checked our primaries for clearly (repeatedly measured) non-solar metallicities using VizieR and found only two metal-poor stars, HD 22879A and HD 49933A, among our new and confirmed CPM systems, respectively. Both are Gaia benchmark stars with respect to metallicity (Jofré et al. 2014) with mean literature [Fe/H] values of −0.85 and −0.39, respectively. Their CPM companions, HD 22879B and HD 49933B, are therefore M subdwarf candidates that can be used for the calibration of M dwarf metallicities (e.g. Neves et al. 2012; Newton et al. 2014).

Our new and confirmed wide CPM companions of nearby F stars distributed all over the sky are good targets for spectroscopic follow-up observations, to verify their spectral types and to confirm their physical association with the primaries by radial velocity measurements. Our lowest-mass new CPM companion, the suspected ≈L3.5 dwarf HD 3861B, is of particular interest, as L-type companions of nearby F-type stars are rare (Wilson et al. 2001; Luhman et al. 2012; Gauza et al. 2012; Deacon et al. 2014). Our spectral type estimate for HD 3861B is also supported by its J−K = +1.7 measured in the UKIDSS LAS, which is a typical colour of a mid-L dwarf (Leggett et al. 2010).

The CPM criterion is also used for membership probability in moving star clusters (e.g. Gagné et al. 2015). One of our new CPM companions, HD 175317B (Table A.1), was previously considered a member of the AB Dor moving group by Malo et al. (2013), who did not mention the small separation (≈18 arcsec) and CPM with respect to HD 175317A. We consider this relatively close CPM pair most likely physically bound, although the μαcosδ agreement is only within 3σ. This does not exclude a moving group membership. The confirmed CPM companion HD 126679B (Table B.1) was investigated by Gagné et al. (2015), but was not found to be a member of any moving group.

A strong decline in the frequency of Sirius-like systems (AFGK stars with WD companions) beyond a distance of 20 pc was mentioned by Holberg et al. (2013), who predicted new discoveries of such systems with different observing techniques. In our search, we confirmed two WD CPM companions (Table B.1) and found one previously overlooked at a separation of about 20 arcsec to the early-F star HD 2726A (Table A.1). The new object, HD 2726B, was barely seen in 2MASS, but was clearly detected in DENIS (J = 14.96, I−J = −0.33) and VISTA VHS (mean J = 14.83, J−K = −0.19). Both of its proper motion components agree to within 1σ with those of HD 2726A.

Our most doubtful CPM confirmation, HD 165670H, is not red enough for an early-M dwarf classification (2MASS J = 10.27, J−Ks = +0.39) and was therefore also considered a WD candidate. Deacon et al. (2014) classified it as DA WD based on a near-infrared spectrum and found a fainter magnitude and bluer colour (UKIRT J = 11.10, J−K = +0.15). Their measured separation of 9.6 arcsec is 1.8 arcsec larger than ours, indicating centroiding problems or a change over time (no CPM?), but their proper motion (+36 ± 5, −133 ± 5) is in better agreement with that of the primary. However, the primary shows a discrepancy between its Hipparcos and UrHip proper motions. More importantly, the J magnitude of HD 165670H is comparable to that of the nearest known WDs (Fig. 1 in Scholz et al. 2015). Therefore, this is either a very nearby WD in the foreground or a different kind of object (hot subdwarf?) associated with HD 165670A.

2

Old: http://irsa.ipac.caltech.edu/applications/FinderChart/ (for DSS, SDSS, and 2MASS 1−6 arcmin image sizes), new: http://irsa.ipac.caltech.edu/applications/finderchart/ (with additional WISE images, and allowing for smaller image sizes).

Acknowledgments

We thank the referee, Andrei Tokovinin, for helpful advice and Jesper Storm for discussion. This research made use of the VizieR catalogue access tool, the SIMBAD database, and the cross-match service provided by CDS Strasbourg, France, he WFCAM Science Archive providing UKIDSS, the VISTA Science Archive, the NASA/IPAC Infrared Science Archive (IRSA), operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration, the SDSS DR12 Science Archive Server, as well as the DENIS and SSS images.

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Appendix A: Newly found CPM companions

Table A.1 presents our new CPM discoveries. Here we include some CPM companions with previous proper motion

measurements (three in UCAC4, four in URAT1) and/or spectral classifications (one early-M dwarf) whose association with the primary was not mentiond before.

Table A.1

Data on nearby F stars and their new CPM companions.

Appendix B: Previously known CPM companion candidates confirmed with new data

In Table B.1 we list F star companions known in the literature with missing or uncertain proper motion measurements. In particular, we include companions previously listed without proper motion in Tokovinin (2014) or in the WDS catalogue (Mason et al. 2001). For some URAT1 CPM companions recently identified by Nicholson (2015), we provide improved proper motion and spectral type estimates. CPM companions of Hipparcos stars already reported by Gould & Chanamé (2004) and Lépine & Bongiorno (2007) are not considered here.

Table B.1

Data on confirmed CPM companions of nearby F stars.

Appendix C: Rejected CPM companions

Table C.1 lists objects that appeared to be CPM companions according to SIMBAD. We have determined new proper motions that are in disagreement with those of the F star primaries.

Table C.1

Data on rejected CPM pairs including F stars.

Appendix D: Selected finder charts

Figure D.1 shows the IRSA finder charts of the new nearby (d = 25.6 pc) CPM pair HD 22879AB. The faint ≈M6/sdM? companion HD 22879B overlaps with the diffraction spike of its primary, the Gaia benchmark metal-poor star HD 22879A, but it was visually measured in the 2MASS images and the photographic IR image taken from the SSS. The red and blue circles show the change in the positions of the components from 1998 (2MASS) to 2013 (URAT1).

thumbnail Fig. D.1

Finder charts of 1 × 1 arcmin2 (north is up, east to the left) from photographic plates of the Digitized Sky Surveys (DSS) and from 2MASS of the CPM pair HD 22879AB. Red circles: first-epoch (1998.732) positions (2MASS) of components A (left) and B (right), blue circles: last-epoch (2012.955) positions (URAT1), green crosses: 2MASS artifacts.

thumbnail Fig. D.2

Top: finder chart of 1 × 1 arcmin2 (north is up, east to the left) from UKIDSS LAS (K-band) centred on HD 3861B. Bottom: 30 × 30 arcsec2 2MASS finder charts of the CPM pair HD 3861AB. Red circles: first-epoch (2000.723) positions (2MASS) of components A (left) and B (right), blue circles: last-epoch (2010.592) positions (UKIDSS LAS), green crosses: 2MASS artifacts.

As a second example, HD 3861AB, which is our new CPM pair containing the latest-type (≈L3.5) companion that was not detected on photographic plates, is shown in Fig. D.2. Again, the companion appeared close to the diffraction spike of its primary and was visually measured in 2MASS. The small 2MASS finder charts illustrate the proper motion of the components by the red and blue circles, whereas the UKIDSS LAS finder chart is centred on HD 3861B.

Finally, we show the IRSA and VHS finder charts of HD 2726AB in Fig. D.3; the new WD companion is clearly seen in two of the photographic images, although it appears very faint in 2MASS. Interestingly, the deeper near-infrared data from the VHS provided the last-epoch data for this rather blue object, thus helping us to confirm the CPM illustrated by the red and blue circles.

thumbnail Fig. D.3

Top: finder chart of 1 × 1 arcmin2 (north is up, east to the left) from VHS (J-band) centred on HD 2726B. Bottom: finder charts of 30 × 30 arcsec2 from photographic plates of the Digitized Sky Surveys (DSS) and from 2MASS of the CPM pair HD 2726AB. Red circles: first-epoch (1992.841) positions (DSS2 IR) of components A (left) and B (right), blue circles: last-epoch (2011.625) positions (VHS), green crosses: 2MASS artifacts.

All Tables

Table A.1

Data on nearby F stars and their new CPM companions.

In the text
Table B.1

Data on confirmed CPM companions of nearby F stars.

In the text
Table C.1

Data on rejected CPM pairs including F stars.

In the text

All Figures

thumbnail Fig. D.1

Finder charts of 1 × 1 arcmin2 (north is up, east to the left) from photographic plates of the Digitized Sky Surveys (DSS) and from 2MASS of the CPM pair HD 22879AB. Red circles: first-epoch (1998.732) positions (2MASS) of components A (left) and B (right), blue circles: last-epoch (2012.955) positions (URAT1), green crosses: 2MASS artifacts.

In the text
thumbnail Fig. D.2

Top: finder chart of 1 × 1 arcmin2 (north is up, east to the left) from UKIDSS LAS (K-band) centred on HD 3861B. Bottom: 30 × 30 arcsec2 2MASS finder charts of the CPM pair HD 3861AB. Red circles: first-epoch (2000.723) positions (2MASS) of components A (left) and B (right), blue circles: last-epoch (2010.592) positions (UKIDSS LAS), green crosses: 2MASS artifacts.

In the text
thumbnail Fig. D.3

Top: finder chart of 1 × 1 arcmin2 (north is up, east to the left) from VHS (J-band) centred on HD 2726B. Bottom: finder charts of 30 × 30 arcsec2 from photographic plates of the Digitized Sky Surveys (DSS) and from 2MASS of the CPM pair HD 2726AB. Red circles: first-epoch (1992.841) positions (DSS2 IR) of components A (left) and B (right), blue circles: last-epoch (2011.625) positions (VHS), green crosses: 2MASS artifacts.

In the text

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