High proper motion objects towards the inner Milky Way: characterisation of newly identified nearby stars from the VISTA Variables in the Via Lactea Survey
M. Gromadzki, R. Kurtev, J.C. Beamin, A. Tekola, R. Ramphul, V.D. Ivanov, D. Minniti, S.L. Folkes, P. Vaisanen, A.Y. Kniazev, J. Borissova, S.G. Parsons, V. Villanueva
aa r X i v : . [ a s t r o - ph . S R ] S e p ACTA ASTRONOMICA
Vol. (2016) pp. 0–0 High proper motion objects towards the inner Milky Way:characterisation of newly identified nearby stars from the VISTAVariables in the Vía Láctea Survey
G r o m a d z k i , , M., K u r t e v , , R., B e a m í n , , J. C., T e k o l a , A.,R a m p h u l , , R., I v a n o v , V. D., M i n n i t i , , , D., F o l k e s , , S. L.,V a i s a n e n , , P., K n i a z e v , , , A. Y., B o r i s s o v a , , J.,P a r s o n s , S. G. and V i l l a n u e v a , , V. Millennium Institute of Astrophysics, Av. Vicua Mackenna 4860, 782-0436, Macul,Santiago, Chile Instituto de Física y Astronomía, Universidad de Valparaíso, Av. Gran Bretaña 1111,Playa Ancha, Casilla 5030, Valparaíso, Chilee-mail:[email protected] Las Cumbres Observatory Global Telescope Network, Inc., 6740 Cortona Drive, Suite102, Goleta, CA 93117, USA South African Astronomical Observatory, P.O. Box 9 7935, South Africa University of Cape Town, Astronomy Department, Rondebosch 7701, South Africa European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching beiMünchen, Germany Departamento de Ciencias Físicas, Universidad Andres Bello, Republica 220, Santiago,Chile Vatican Observatory, V00120 Vatican City State, Italy Centre for Astrophysics Research, Science and Technology Research Institute,University of Hertfordshire, Hatfield AL10 9AB, UK Southern African Large Telescope Foundation, P.O. Box 9 7935, South Africa Sternberg Astronomical Institute, Lomonosov Moscow State University, Moscow,Russia Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK
Received June 10, 2016
ABSTRACTThe census of the Solar neighbourhood is still incomplete, as demonstrated by recent discov-eries of many objects within 5–10 pc from the Sun. The area around the mid-plane and bulge ofthe Milky Way presents the most difficulties in searches for such nearby objects, and is thereforedeficient in the known population. This is largely due to high stellar densities encountered. Spectro-scopic, photometric and kinematic characterization of these objects allows better understand the localmass function, the binary fraction, and provides new interesting targets for more detailed studies. Wereport the spectroscopic follow-up and characterisation of 12 bright high PM objects, identified from ol. 66 the VISTA Variables in Vía Láctea survey (VVV). We used the 1.9-m telescope of the South AfricanAstronomical Observatory (SAAO) for low-resolution optical spectroscopy and spectral classifica-tion, and the MPG/ESP 2.2m telescope Fiber-fed Extended Range Optical Spectrograph (FEROS)high-resolution optical spectroscopy to obtain the radial and space velocities for three of them. Sixof our objects have co-moving companions. We derived optical spectral types and photometric dis-tances, and classified all of them as K and M dwarfs within 27 – 264 pc of the Sun. Finally, we foundthat one of the sources, VVV J141421.23-602326.1 (a co-moving companion of VVV J141420.55-602337.1), appears to be a rare massive white dwarf that maybe close to the ZZ Ceti instability strip.Many of the objects in our list are interesting targets for exoplanet searches. Key words: proper motions – stars: low-mass – (stars:) white dwarfs – (stars:) binaries: visual –(Galaxy:) solar neighbourhood – techniques: spectroscopic.
1. Introduction
M-dwarfs account for over 70% of stellar systems in the solar vicinity (Henry et al. ∼ et al. et al. ∼
54% for solar-typestars (Duquennoy & Marcy 1991; Raghavan et al. ∼
0% for massivestars (Preibisch et al. e.g.
M2K; Apps et al. et al. ∼
70 pc (Chauvin et al. ∼ . et al. l > µ m(Lépine & Gaidos 2011) published an all-sky catalog of M-dwarfs with appar-ent near-InfraRed (near-IR) magnitude J <
10. They selected 8889 stars fromthe on-going SUPERBLINK ( e.g.
Lépine & Shara 2005) survey of stars with µ >
40 mas yr − , supplemented at the bright end with the TYCHO-2 catalogue. Re-cently, Lepine et al. (2013) presented a spectroscopic catalog of the 1564 brightest( J <
9) M-dwarf candidates in the northern sky.The majority of surveys avoid Galactic plane and bulge, or are substantiallyincomplete near to these regions. However, these regions offer considerable latentpotential for new discoveries of nearby low-mass stars and brown dwarfs. This isespecially true for nearby or bright examples that have been overlooked in previoussearchs due to confusion caused by high stellar densities and background contami-
A. A. nant objects (Folkes et al. et al. (2013) and Ivanov et al. (2013), to generate a uniform catalog of high-PM objects within the VVV footprint,and characterise them with spectroscopic follow-up observations. It is organised asfollows: the next section describes the sample selection, and the new observations.Section 3 describes the spectral type estimation, the distance measurements, andreports on the co-moving companions. Finally, in section 4 we present summaryand conclusions.
2. Sample selection and observations
The targets reported here were identified during a test phase of our method tosearch for high PM objects using the VVV survey database. The VVV observa-tions at the time of the search covered the period from Feb 2010 to Mar 2013.Our PM analysis techniques makes use of four K S -band epochs, preferentially se-lected with equally spaced epochs, obtained under similar seeing and photometricconditions. We used the source catalogs generated by the Cambridge AstronomicalSurvey Unit (CASU) . Their astrometric precision is 0.05–0.09 arcsec. Pairs of cat-alogs for neighbouring epochs were cross-identified with a matching radii scaledto correspond to a maximum PM of 5 arcsec yr − using the STILTS code (Taylor2006). This procedure was repeated for all sequential pairs of epochs, yieldingthree PM measurements for each object. Next, we removed candidates with incon-sistent PMs. At this stage of the project we were interested in nearby objects withlarger apparent motions, so we imposed two additional constraints selecting onlybrightest stars with K S < − . The resulting catalog anda more detailed description of the search will be reported in Kurtev et al. (2016).For this paper we selected 12 high-PM stars based on their magnitudes and thevisibility at the moment of the observations. Four of our targets have been previ-ously identified as high PM stars: VVV J121051.57-642528.5 and VVV J164622.06-420118.8 were listed in TYCHO-2 catalog (Høg et al. et al. http://casu.ast.cam.ac.uk/surveys-projects/vista ol. 66 Low-resolution spectra of 12 stars were acquired on 2013 Apr 4–7 using theGrating Spectrograph with the SITe (Scientific Imaging Technologies, INC.) CCDmounted on the 1.9-m Radcliffe telescope at the South African Astronomical Ob-servatory (SAAO), Sutherland. We made use of grating no. 7 with 300 lines mm − and a slit with a projected width of 1.35 arcsec. During observations seeing var-ied from 0.8 to 2.5 arcsec, with an average around 1.5 arcsec. The total durationof an exposure was 600-1200 s, depending on object’s brightness, split into twoindividual integrations. Usually, two or three standards were observed on eachnight, selected among the list: LTT 3864, LTT 4816, LTT 7379, LTT 6248, andCD − ◦ procedures. The final spectra from the first two nights cover the range4292–8289Å and from the last two nights 3761–7735Å with a resolving power ofR ∼ We obtained radial velocities (RV) for VVV J121436.36-640808.4, VVV J132355.14-620324.9 and VVV J164810.92-414014.9 with the Fiber-fed Extended Range Op-tical Spectrograph (FEROS; Kaufer et al. et al. (2014) and Brahm, Jordán andEspinoza (2016) for a detailed explanation of each step. First, the bias was re-moved, and the flat fielding correction was applied. Next, one-dimensional spectrawere optimally extracted for each echelle order of both the science and the calibra-tion spectra. The wavelength calibration is first processed order by order using areference Thorium-Argon lamp, each order is fitted against this reference until theR.M.S. is less than 70 m s − then a global solution is performed iteratively untilan R.M.S. below 100 m s − is reached. Then, the solution is applied to the targetspectrum. Finally, a barycentric correction is applied using the Jet Propulsion Lab IRAF is distributed by the National Optical Astronomy Observatory, which is operated by theAssociation of Universities for Research in Astronomy (AURA) under cooperative agreement withthe National Science Foundation.
A. A. ephemerides (JPLEphem) package .The reduced spectra are cross correlated with a set of synthetic spectra fromCoelho et al. (2005) to estimate the physical parameters in a iterative way, andafter convergence a binary mask is used to measure the RV via the cross correlationfunction (Baranne et al. et al. VISTA Variables in the Vía Láctea (VVV) is a public ESO (European SouthernObservatory) near-infrared (near-IR) survey that is mapping the Milky Way Bulgeand an adjacent section of the mid-plane with the VISTA telescope (Minniti et al. et al. ∼ × tiles, working in the 0.9–2.4 µ m wavelength range. TheVISTA data are processed with the VISTA Data Flow System (VDFS; Irwin et al. et al. et al. in the Galactic bulge andsouthern disk. The VVV database now contains multicolour photometry in ZY JHK S -bands, and multiple epochs in the K S -band, monitoring a billion sources in total(Saito et al. e.g. Hempel et al. ∼
25 mas for a K s = ∼
175 mas for K s = ∼
10 mas yr − ( K s = ∼
20 mas yr − ( K s = et al. (2012). We have just started to exploit theVVV database for proper motion studies: Beamín et al. (2013) reported the firstVVV brown dwarf discovery, Ivanov et al. (2013) found seven new companionsto known high PM nearby stars, Libralato et al. (2015) produced a high-precisionastrometry reduction pipeline for the VVV survey data, and Kurtev et al. (2016)produced a catalogue of 3 003 high proper motion stars in the VVV fields. https://pypi.python.org/pypi/jplephem ol. 66 We complemented the VVV observations with archival multi-wavelength pho-tometry to obtain their spectral energy distributions. Photometric data for our ob-jects were found in the TYCHO-2 catalog Høg et al. (2000), the Fourth U.S. NavalObservatory CCD Astrograph Catalogue (UCAC4; Zacharias et al. et al. et al. et al. et al. et al. et al.
3. Results
We derived spectral type for our targets using three methods: direct comparisonwith spectral templates, spectral indices, and spectral energy distributions (SEDs)fitting. We find good agreement between spectral types determined with the firsttwo methods. The third method yields results, that sometimes differ by up to fivesub-types. We attribute the poor agreement to the significant background contam-ination of the archival photometry – both for old photographic surveys, and themid-infrared WISE and Neo-WISE surveys. We adopted as our final spectral typesthose obtained by the templates comparison, as the most direct and robust and adoptuncertainties of their estimation equal one sub-type.
Nine of the twelve spectra showed the characteristics molecular absorption bands ofK and M type stars (TiO, CaH), and they were compared to the primary K7V-M5Vstandards from Kirkpatrick et al. (1991, 1999) available from the Dwarf Archives .The template spectra were smoothed to the resolution of our data, normalisedat 7500 Å, and a c minimisation over l =5000–8000 Å was used to find the bestmatch. The results are shown in Fig. 1. The types of some targets were adjustedby up to 0.5 sub-type after a visual inspection. The remaining three spectra in-dicated hotter stars, and for those earlier than K7 type objects we used the stan-dards of Pickles (1998), following the same minimisation routine as before, butnow over l =3800–8000 Å. In case of VVV J121051.57-642528.5, we cannot find A. A. a convenient fit to the low resolution spectrum (see Fig. 1). Target was overexposeand detector worked in nonlinear regime, what prevented proper flux calibration.Spectral type of this object was estimated by comparison of Na I doublet around5890 Å in the high resolution spectrum with template spectrum. Both spectra werenormalised by continuum. Fe I lines around Na I doublet fit perfectly what suggestsimilar rotation velocity. The derived spectral types are listed in Table 2.
Wavelength [ ◦ A] VVV J121051.57-642528.5 K3G5VVV J121436.36-640808.4 K7VVV J122701.70-634203.7 M0K7VVV J130523.07-620002.4 M1M0VVV J132355.14-620324.8 M2.5VVV J140829.88-594823.5 M1M0
Wavelength [ ◦ A]VVV J141420.55-602337.1 M2.5M2VVV J141513.65-591356.1 K5VVV J141718.52-595754.9 M2VVV J145009.14-603603.8 M4M5VVV J164622.06-420118.7 K2VVV J164810.92-414014.8 M3M2.5 N o r m a li z e d F l u x Fig. 1. Observed spectra (black lines) overplotted with the best fitted template spectra (red lines)and alternative comparison template spectra (green lines). The template spectra were taken fromKirkpatrick et al. (1991) and Pickles (1998) for objects with spectral types later, and earlier than K5,respectively. The absorption feature at ∼ Kirkpatrick et al. (1991) developed a system of spectroscopic indices for mid-K tolate-M stars, and calibrated them versus the spectral types. Other index systemshave been developed afterwords, but we prefer this one because it has been applied ol. 66 et al. (1991). Our formal Poisson errors are negligible, and we also verifiedthat a small velocity offset of ±
20 km s − led to changes in the ratios of ≤ et al. T a b l e 1
Spectroscopic index ratios (as defined by Kirkpatrick et al. − < K8 1.034 K9VVV J121436.36 − ± M2 1.133 M3VVV J122701.70 − ± < K8VVV J130523.07 − ± M2 1.163 M4VVV J132355.14 − ± < K8 1.101 M2VVV J140829.88 − ± ± M2VVV J141420.55 − ± M3VVV J141513.65 − K9 1.044 M3VVV J141718.52 − ± M2VVV J145009.14 − ± ± − < K8 1.005 K9 1.091 M2VVV J164810.92 − ± ± To produce the spectral energy distribution (SED) and compare to stellar models weused the Virtual Observatory SED analyser (VOSA; Bayo et al. et al. et al. c minimisation over three parameter space:effective temperature T eff , surface gravity log g , and metallicity [Fe/H]. Increasingthe number of photometric measurements and widening the wavelength coveragemakes the fit more robust and reliable. Usually, the T eff is the most stringently A. A. constrained parameter, with uncertainties of order of ∼
200 K; The other parame-ters surface gravity and iron abundance, are usually, log g ≥ ≥ − T a b l e 2
Spectral classifications (based on template comparison) and effective temperatures defined basedon photometric spectral energy distribution, J -band magnitudes, proper motions (estimated basedon 2MASS and the last available epoch of the VVV survey), spectro-photometric distances, andtangential velocities for the observed targets.Name Sp. T. SED T eff J µ a cos d µ d Dist. V tan [K] [mag] [mas yr − ] [mas yr − ] [pc] [km s − ]VVV J121051.57-642528.5 K2 5300 8.211 -118 ±
10 -47 ±
10 55 33 ± ± ± ± ±
10 -35 ±
10 264 227 ± ± ± ± ± ± ± ±
11 -32 ±
11 68 53 ± ±
10 -65 ±
10 84 78 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± We were not able to measure parallaxes from the multi-epoch K S -band VVVdata as achieved by Beamín et al. (2013, 2015) and Smith et al. (2015). This is dueto our targets being saturated on the VVV K s images. Instead, we estimated theirspectro-photometric distances from the spectral types we obtain (see Sec. 3.1), andfrom the 2MASS J and K S photometry using the absolute magnitudes M J , M K S from Pecaut & Mamajek (2013) . For each object we calculated the distances forthe two 2MASS photometric bands separately. Our final estimate was the averageof these two measurements. The mean difference is 1.6% ± ∼ ∼ emamajek/EEM_dwarf_UBVIJHK_ colors_Teff.txt ol. 66 Six of our objects have co-moving companions. We estimated the T eff of thecompanions with SED fitting (see Sec. 3.1..3; the photometry is available in onlinematerials We also derived spectral types of companions using magnitude differencewith the primary in J band and the absolute magnitudes for a given spectral typefrom Pecaut & Mamajek (2013). The basic information about the binary systemsare summarised in Table 3. T a b l e 3
Binary systems investigated in this paper. The columns are: spectral types from this work, T eff defined based on photometric spectral energy distribution, 2MASS J -band magnitudes, angularseparations in arcsec, photometric distances in pc, and proper motions in celestial coordinates.Name Sp. T. SED T eff J r Dist. r p µ a cos d µ d [K] [mag] [”] [pc] [a.u.] [mas yr − ] [mas yr − ]VVV J121051.57-642528.5 K2 5300 8.211 45.8 55 2521 -118 ±
10 -47 ± a ±
10 -49 ± ± ± a ± ± ±
11 -32 ± a ±
11 -31 ± ±
10 -65 ± ±
10 -78 ± ± ± a - 11.845 b - - - -143 ± ± ± ± a ± ± a Sp. types of companions estimated base on magnitude differences and absolute magnitudes givenby Pecaut & Mamajek (2013). b System not resolved in achival images, magnitudes were takenfrom VVV catalogs and then transform to the 2MASS system.
Finch et al. (2010) reported that the object UPM 1414 − − − − ZY JHK s magnitudes (transforming the JHK S to the 2MASS system follow-0 A. A. ing Soto et al. I -band, and compare them with the syntheticcolour of pure hydrogen white dwarfs form Pierre Bergeron webpage (Holberg &Bergeron 2006; Kowalski & Saumon 2006; Tremblay et al. et al. − eff ∼
12 000 K and a relativelyhigh mass of ∼ M ⊙ (Fig. 2). These parameters place it in the the ZZ Ceti instabil-ity strip. Typically, the ZZ Ceti pulsators have log g ∼ et al. ∼ M ⊙ so, VVV J141421.23 − M ⊙ within the approved ZZ Cetis, and only three ofthem are more massive than 1 M ⊙ (Castanheira & Kepler 2014). Each new memberof this tiny family is valuable, because the larger number of massive pulsators willallows to probe the ensemble internal structure of the high-mass end of the ZZ Cetiinstability strip.Further spectroscopic and photometric follow-up is needed to confirm natureand to determine the age of this object. High time resolution photometry coulddetect its variability and if it is indeed a ZZ Ceti star, providing us with additionalconstraints about its structure and composition.
11 11.5 12 12.5 13 13.5-0.3 -0.2 -0.1 0 M I I-J
11 11.5 12 12.5 13 13.5-0.3 -0.2 -0.1 0 M I I-J
11 11.5 12 12.5 13 13.5-0.3 -0.2 -0.1 0 M I I-J
11 11.5 12 12.5 13 13.5-0.3 -0.2 -0.1 0 M I I-J
11 11.5 12 12.5 13 13.5-0.3 -0.2 -0.1 0 M I I-J -0.1-0.05 0 0.05 0.1-0.3 -0.2 -0.1 0 J - H I-J -0.1-0.05 0 0.05 0.1-0.3 -0.2 -0.1 0 J - H I-J -0.1-0.05 0 0.05 0.1-0.3 -0.2 -0.1 0 J - H I-J -0.1-0.05 0 0.05 0.1-0.3 -0.2 -0.1 0 J - H I-J -0.1-0.05 0 0.05 0.1-0.3 -0.2 -0.1 0 J - H I-J
Fig. 2. Position of our new massive ZZ Ceti type pulsator candidate, VVV J141421.23-602326.1,in the colour-magnitude diagram (left panel) and colour–colour diagram (right panel). The black dotrepresents the measurements of VVV J141421.23-602326.1 along with the 3-sigma errorbars. Theblack dashed lines represent the sythetic colours for a given mass for a pure hydrogen atmoshperemodel. The blue dashed lines show the model isotherms. The red solid lines present borders ofZZ Ceti instability strip taken from Gianninas et al. (2011). ∼ bergeron/CoolingModels ol. 66 We measured radial velocities for three objects: VVV J121436.36-640808.4,VVV J132355.14-620324.9 and VVV J164810.92-414014.9, using FEROS spec-trograph at the MPG/ESP 2.2m telescope (Sec. 2.2). The
UWV galactic space ve-locities were computed with GAL_UVW routine from the IDL Astronomy User’sLibrary. The results are listed in Table 4. We used the BANYAN II web tool (Gagné et al. et al. b Pictoris moving group. Although, high resolution spectrum of this targetdoes not show Li I doublet at ∼ T a b l e 4
Radial velocities RV and
UVW galactic space velocities. All velocities are expressed in km s − .VVV Name RV U V W
J121051.57-642528.5 3.27 ± ± ± ± ± ± ± ± ± ± ± ±
4. Summary and conclusions
We obtained spectroscopic follow-up observations of twelve new high PM ob-jects found by the VVV survey during the initial testing of our searching method,and we also looked for possible new wide binary companions. We derived theiroptical spectral types and photometric distances.All of the analysed objects are K and M dwarfs located at 27–264 pc from theSun and are bright enough for further follow-up and search of planets using state ofthe art and upcoming NIR instruments. Also, all objects are in the densest regionsof the Milky Way, surrounded by a pletora of bright stars, very suitable for AOimaging. That makes our targets ideal for searches of close neighbours. From ∼ gagne/banyanII.php A. A. the other side, the surrounded stars are ideal comparison stars for precise relativephotometry, variability and transit studies.VVV J141421.23-602326.1, a co-moving companion of VVV J141420.55-602337.1,is a candidate for being a rare massive ZZ Ceti type pulsator. Further spectroscopicand photometric follow-up is needed to better constrain nature and age of this ob-ject.
Acknowledgements.
We gratefully acknowledge use of data from the ESOPublic Survey programme ID 179.B-2002 taken with the VISTA telescope, anddata products from the Cambridge Astronomical Survey Unit. This publicationmakes use of data products from the Two Micron All Sky Survey, which is a jointproject of the University of Massachusetts and the Infrared Processing and Anal-ysis Center/California Institute of Technology, funded by NASA and NSF.Thisresearch has benefitted from the M, L, T, and Y dwarf compendium housed atDwarfArchives.org. Support for MG, RK, JCB, DM, and JB is provided by theMinistry of Economy, Development, and Tourisms Millennium Science Initiativethrough grant IC120009, awarded to The Millennium Institute of Astrophysics,MAS. MG acknowledges support from Joined Committee ESO and Government ofChile 2014. RK, DM and JB are supported by FONDECYT grants No. 1130140,1130196 and 1120601, respectively. Both PV and AYK acknowledge support fromthe National Research Foundation of South Africa. JB, RK, MG and VV are sup-ported by CONICYT REDES140042. JCB acknowledge support from CONICYTFONDO GEMINI - Programa de Astronomía del DRI, Folio 32130012.REFERENCES
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