Proper motions of Upper Sco T-type candidates
MMon. Not. R. Astron. Soc. , 1– ?? (2005) Printed 16 October 2018 (MN L A TEX style file v2.2)
Proper motions of USco T–type candidates (cid:63) † N. Lodieu , ‡ , V. D. Ivanov , & P. D. Dobbie Instituto de Astrof´ısica de Canarias (IAC), C/ V´ıa L´actea s/n, E-38200 La Laguna, Tenerife, Spain Departamento de Astrof´ısica, Universidad de La Laguna (ULL), E-38205 La Laguna, Tenerife, Spain European Southern Observatory, Santiago de Chile, Chile School of Mathematics & Physics, University of Tasmania, Hobart, TAS, 7001, Australia
Accepted 16 October 2018. Received 16 October 2018; in original form 16 October 2018
ABSTRACT
We present new z - and H -band photometry and proper motion measurements for the fivecandidate very-low-mass T–type objects we recently proposed to be members of the nearestOB association to the Sun, Upper Scorpius. These new data fail to corroborate our prior con-clusions regarding their spectral types and affiliation with the Upper Scorpius population. Weconclude that we may be in presence of a turnover in the mass function of Upper Sco takingplace below 10–4 Jupiter masses, depending on the age assigned to Upper Sco and the modelsused. Key words:
Stars: low-mass stars and brown dwarfs — techniques: photometric — Infrared:Stars — surveys — stars: luminosity function, mass function
The quest for young objects of spectral-type T remains an areaof substantial interest as a way to address a fundamental questionin our understanding of star formation: what is the lowest massthat this process can form ? The earliest theoretical predictions byKumar (1969), Low & Lynden-Bell (1976), and Rees (1976) sug-gested masses as low as ∼
10 Jupiter (M
Jup ) but contemporary cal-culations reveal that in the presence of magnetic fields this limitcould be much lower (Boss 2001; Stamatellos & Whitworth 2008).Naturally, the searches for these objects have concentrated onthe nearest young clusters and star-forming regions and these haveled to the identification of several candidate infantile T–type ob-jects. Crucially, none of these has been unambiguously confirmedastrometrically and spectroscopically. For example, Bihain et al.(2010) have detected a further candidate T–type member of the σ Ori cluster, adding to the previously known candidate mid-T,S Ori 70 (Zapatero Osorio et al. 2002, 2008; Burgasser et al. 2004;Scholz & Jayawardhana 2008; Luhman et al. 2008; Zapatero Os-orio et al. 2008). However, proper motion measurements of bothobjects cast doubt on their association with this population (Pe˜naRam´ırez et al. 2011). More recently, Pe˜na Ram´ırez et al. (2012)have identified another candidate T–type in this same region us-ing photometry from the VISTA (Visible and Infrared Survey Tele-scope for Astronomy; Emerson et al. 2004) Orion survey.Additionally, Marsh et al. (2010) have claimed the discovery (cid:63)
Based on observations collected with the ESO New Technology Tele-scope under programme number 089-C.0854(A). † Based on observations made with the Gran Telescopio Canarias (GTC),installed in the Spanish Observatorio del Roque de los Muchachos of theInstituto de Astrofsica de Canarias, in the island of La Palma. ‡ E-mail: [email protected] of a T2 member of ρ Ophiuchus but this has since been refuted byAlves de Oliveira et al. (2010). Independently, Geers et al. (2011)has proposed several candidates as substellar members of this pop-ulation through infrared spectroscopy, including one with a massclose to the deuterium burning limit. Another wide-field methaneimaging survey of ρ Ophiuchus revealed 22 T–type dwarf candi-date members down to 1–2 Jupiter (Haisch et al. 2010). Burgesset al. (2009) identified a mid-T–type candidate from a deep methanesurvey of ∼ ∼
50 Jupiter masses) than the young T–types dueto their substantially greater ages, τ ∼
600 Myr.Upper Scorpius (hereafter USco) is part of the Scorpius Cen-taurus OB association: it is located at 145 pc from the Sun (de Brui-jne et al. 1997) and its age is estimated to 5 ± ± c (cid:13) a r X i v : . [ a s t r o - ph . S R ] M a r N. Lodieu lengths. Tens of brown dwarfs have now been confirmed spectro-scopically as members of USco (Mart´ın et al. 2004; Slesnick et al.2006; Lodieu et al. 2006; Slesnick et al. 2008; Lodieu et al. 2008;Mart´ın et al. 2010; Lodieu et al. 2011) and the mass function ofthis population determined robustly, deep into the substellar regime(Preibisch et al. 2002; Slesnick et al. 2008; Lodieu et al. 2011).In a recent paper in our extensive series of publications re-lating to USco (Lodieu et al. 2011) we identified five T–type can-didate members with deep infrared photometry from the UK In-frared telescope wide-field camera (UKIRT/WFCAM; Casali et al.2007). In the current work, we report proper motion measurementsfor these objects obtained from early deep WFCAM J -band ob-servations and new H -band imaging that are separated in time byfour years. In Section 2 and Section 3 we describe the new H -bandobservations carried out with the Son of Isaac (SofI) instrumentinstalled on the European Southern Observatory (ESO) New Tech-nology Telescope (NTT) in La Silla Observatory (Chile) and addi-tional z -band imaging conducted with the Optical System for Imag-ing and low Resolution Integrated Spectroscopy (OSIRIS) installedon the Gran Telescopio de Canarias (GTC) in La Palma Observa-tory (Canary Islands). In Section 4 we use the new photometry andastrometry to examine the membership status of the five candidateT–type members. In Section 5 we place our new results into contextand speculate about our (positive/negative) results. H -band imaging We performed near-infrared imaging of the five T–type candidatesin USco listed in Table 4 of Lodieu et al. (2011) in the H filter withSofI on the 3.5-m NTT (Moorwood et al. 1998). All five sourceswere observed on 9 May 2012.SofI is equipped with a Hawaii HgCdTe 1024 × × The data were reduced with the ESO EXOREX SofI pipelinerecipes. These perform an automatic reduction of the target frameswithin an observing block, including flat field correction, sky sub-traction, and cross-talk removal. The next step of the data analysisincluded photometric and astrometric calibrations using the WF-CAM images as reference.To astrometrically calibrate the SofI images we proceeded as follows: for a first guess we used the astrometry.net package whichrequires the centre of image given by the (RA,dec) coordinates inthe header, the pixel scale (0.292 arcsec/pixel), and a radius for thesearch (set to 12 arcmin, more than twice the field-of-view of theSofI images). The astrometric solution was satisfactory comparingwith 2MASS and the deep WFCAM images obtained as first epoch.However, it was not good enough for our purposes, i.e. to measureproper motions between the two epochs.The second step made use of the GAIA software which it-self uses SExtractor (Bertin & Arnouts 1996). We ran the detec-tion algorithm to extract all sources (pixel and world coordinatessystems) in the SofI images. Then, we cross-correlated this SEx-tractor catalogue against the deep WFCAM dataset and kept onlythe SofI (x,y) and WFCAM (RA,dec) coordinates in an output filefor sources with J -band magnitudes in the 19–20 range. Next weused the IRAF task ccmap interactively with a polynomial of orderfour. Using the faintest stars from the WFCAM images allowed usto exploit more than 100–170 point sources with a small intrinsicmotion on the sky (i.e. about 21–25% of all stars in the each SofIfield), avoiding bright members of the association. We eliminatedpoints whose astrometry was off by more than 5 σ , yielding an rmsof 44.8–51.1 mas and 33.2–45.1 mas in right ascension and decli-nation, respectively (corresponding to about 1/6 of the SofI pixelscale or 11–13 mas/yr). The new image was saved and SExtractorran again with a detection threshold of 3 σ and an aperture twicethe size of the full-width-half-maximum ( ∼ We could not use point sources within the 2MASS database to cal-ibrate photometrically the SofI frames because most of these weresaturated in our images. Instead, we cross-matched all objects de-tected by SExtractor (see Section 2.2) with the ninth data releaseof the UKIDSS GCS and retrieved all point sources detected in H with photometric error bars less than 0.1 mag for each individualfield. The total numbers of matched sources within a matching ra-dius of two arcsec was typically 200–240. We find a median offsetof − ± H magnitudes and their errors of our five USco targets,computed using the offsets from each individual frame. z -band imaging OSIRIS is the Optical System for Imaging and low Resolution In-tegrated Spectroscopy instrument (Cepa et al. 2000) on the 10.4-mGTC operating at the Observatory del Roque de Los Muchachos(La Palma, Canary Islands). The OSIRIS instrument is equipped More details at astrometry.net GAIA is a derivative of the Skycat catalogue and image display tool,developed as part of the VLT project at ESO. Skycat and GAIA are freesoftware under the terms of the GNU copyright.c (cid:13) , 1–, 1–
600 Myr.Upper Scorpius (hereafter USco) is part of the Scorpius Cen-taurus OB association: it is located at 145 pc from the Sun (de Brui-jne et al. 1997) and its age is estimated to 5 ± ± c (cid:13) a r X i v : . [ a s t r o - ph . S R ] M a r N. Lodieu lengths. Tens of brown dwarfs have now been confirmed spectro-scopically as members of USco (Mart´ın et al. 2004; Slesnick et al.2006; Lodieu et al. 2006; Slesnick et al. 2008; Lodieu et al. 2008;Mart´ın et al. 2010; Lodieu et al. 2011) and the mass function ofthis population determined robustly, deep into the substellar regime(Preibisch et al. 2002; Slesnick et al. 2008; Lodieu et al. 2011).In a recent paper in our extensive series of publications re-lating to USco (Lodieu et al. 2011) we identified five T–type can-didate members with deep infrared photometry from the UK In-frared telescope wide-field camera (UKIRT/WFCAM; Casali et al.2007). In the current work, we report proper motion measurementsfor these objects obtained from early deep WFCAM J -band ob-servations and new H -band imaging that are separated in time byfour years. In Section 2 and Section 3 we describe the new H -bandobservations carried out with the Son of Isaac (SofI) instrumentinstalled on the European Southern Observatory (ESO) New Tech-nology Telescope (NTT) in La Silla Observatory (Chile) and addi-tional z -band imaging conducted with the Optical System for Imag-ing and low Resolution Integrated Spectroscopy (OSIRIS) installedon the Gran Telescopio de Canarias (GTC) in La Palma Observa-tory (Canary Islands). In Section 4 we use the new photometry andastrometry to examine the membership status of the five candidateT–type members. In Section 5 we place our new results into contextand speculate about our (positive/negative) results. H -band imaging We performed near-infrared imaging of the five T–type candidatesin USco listed in Table 4 of Lodieu et al. (2011) in the H filter withSofI on the 3.5-m NTT (Moorwood et al. 1998). All five sourceswere observed on 9 May 2012.SofI is equipped with a Hawaii HgCdTe 1024 × × The data were reduced with the ESO EXOREX SofI pipelinerecipes. These perform an automatic reduction of the target frameswithin an observing block, including flat field correction, sky sub-traction, and cross-talk removal. The next step of the data analysisincluded photometric and astrometric calibrations using the WF-CAM images as reference.To astrometrically calibrate the SofI images we proceeded as follows: for a first guess we used the astrometry.net package whichrequires the centre of image given by the (RA,dec) coordinates inthe header, the pixel scale (0.292 arcsec/pixel), and a radius for thesearch (set to 12 arcmin, more than twice the field-of-view of theSofI images). The astrometric solution was satisfactory comparingwith 2MASS and the deep WFCAM images obtained as first epoch.However, it was not good enough for our purposes, i.e. to measureproper motions between the two epochs.The second step made use of the GAIA software which it-self uses SExtractor (Bertin & Arnouts 1996). We ran the detec-tion algorithm to extract all sources (pixel and world coordinatessystems) in the SofI images. Then, we cross-correlated this SEx-tractor catalogue against the deep WFCAM dataset and kept onlythe SofI (x,y) and WFCAM (RA,dec) coordinates in an output filefor sources with J -band magnitudes in the 19–20 range. Next weused the IRAF task ccmap interactively with a polynomial of orderfour. Using the faintest stars from the WFCAM images allowed usto exploit more than 100–170 point sources with a small intrinsicmotion on the sky (i.e. about 21–25% of all stars in the each SofIfield), avoiding bright members of the association. We eliminatedpoints whose astrometry was off by more than 5 σ , yielding an rmsof 44.8–51.1 mas and 33.2–45.1 mas in right ascension and decli-nation, respectively (corresponding to about 1/6 of the SofI pixelscale or 11–13 mas/yr). The new image was saved and SExtractorran again with a detection threshold of 3 σ and an aperture twicethe size of the full-width-half-maximum ( ∼ We could not use point sources within the 2MASS database to cal-ibrate photometrically the SofI frames because most of these weresaturated in our images. Instead, we cross-matched all objects de-tected by SExtractor (see Section 2.2) with the ninth data releaseof the UKIDSS GCS and retrieved all point sources detected in H with photometric error bars less than 0.1 mag for each individualfield. The total numbers of matched sources within a matching ra-dius of two arcsec was typically 200–240. We find a median offsetof − ± H magnitudes and their errors of our five USco targets,computed using the offsets from each individual frame. z -band imaging OSIRIS is the Optical System for Imaging and low Resolution In-tegrated Spectroscopy instrument (Cepa et al. 2000) on the 10.4-mGTC operating at the Observatory del Roque de Los Muchachos(La Palma, Canary Islands). The OSIRIS instrument is equipped More details at astrometry.net GAIA is a derivative of the Skycat catalogue and image display tool,developed as part of the VLT project at ESO. Skycat and GAIA are freesoftware under the terms of the GNU copyright.c (cid:13) , 1–, 1– ?? roper motions of Upper Sco T–type candidates Table 2.
Photometry for the USco T–type candidates: the
Y, J and methane photometry is from Lodieu et al. (2011) to which we added the new H -bandphotometry from NTT/SofI and z -band magnitudes from GTC/OSIRIS. The z − J and J − H colours are given as well. The resulting proper motionsmeasured between the first and second epoch images at near-infrared wavelengths are quoted in mas/yr.R.A. Dec Y J CH CH H z J − H z − J µ α cos δ µδ µ hh:mm:ss.ss ◦ : (cid:48) : (cid:48)(cid:48) mag mag mag mag mag mag mag mag mas/yr mas/yr mas/yr16:08:35.98 − ± ± ± ± ± ± ± ± − + − ± ± ± ± ± ± ± ± − + − ± ± ± ± ± ± ± ± + − − ± ± ± ± ± ± ± ± + − − ± ± ± ± ± ± ± ± + + Figure 1.
GTC/OSIRIS z -band images for the five candidates (circled and centered). North is up and East is left. Images are 1 (cid:48) × (cid:48) . Table 1.
Offsets between the NTT/SofI and UKIRT/WFCAM H -band pho-tometry using >
100 point sources in each individual SofI field. The lastrow indicates the mean (Avg) value of the offset, taking into account thedispersion and errors on the individual offsets.Field R.A. Dec Offset ( H ) ◦ : (cid:48) : (cid:48)(cid:48) mag1 16:08:35.98 − − ± − − ± − − ± − − ± − − ± − ± with two 2048 × × × z filter available on OSIRIS during May 2012. Bias and skyflatswere observed on 26 May (evening) and 28 +
30 May (morning).On 27 May 2012, we obtained three series of 10 frames with 60 secon-source integrations covering the targets 16084780 − − − − − − − − ◦ on 29 May 2012. We reduced the OSIRIS Sloan z -band images in a standard man-ner under the IRAF environment (Tody 1986, 1993). First, wesubtracted the mean bias and divided by the normalised averagedmaster skyflat to each individual science frame. Then, we combinedeach set of 10 images taken without dithering and finally combinedthose sets applying the offsets to create a master science frame. Wenote that our targets were located on CCD ds9 (Joye & Mandel 2003). First, wesaved in a file a list of point sources from the 2MASS catalogue(Cutri et al. 2003; Skrutskie et al. 2006) spread over the full OSIRISfield-of-view. Second, we ran the daofind task with the adequatedetection and threshold parameters to identify (roughly) the samepoint sources to cross-match them in a subsequent step with the ccxymatch routine. The latter task required a reference star withpixel (x,y) and world coordinate system (ra,dec) coordinates to ef-ficiently cross-match the 2MASS (x,y) and (ra,dec) catalogues. Wetypically found 80–100 stars in the field-of-view of CCD ccmap with a polynomial of order four, resulting in an as-trometric calibration better than 0.1–0.15 arcsec. The final reduced z -band images of the five candidates are shown in Fig. 1. IRAF is distributed by the National Optical Astronomy Observatories,which are operated by the Association of Universities for Research inAstronomy, Inc., under cooperative agreement with the National ScienceFoundationc (cid:13) , 1– ?? N. Lodieu
The GTC calibration plan provided us with only one observationof a photometric standard star (G 163-50) taken on the night of 27May 2012 with a single on-source integration of 0.8 sec at an air-mass of 1.253. This DA3.2 white dwarf (Holberg et al. 2012) isa Sloan photometric standard (Adelman-McCarthy & et al. 2011)and has a z -band magnitude of 13.809. We measured the instru-mental magnitude using aperture photometry and applied a curve-of-growth analysis to allow for all the flux from the standard star.We obtained a photometric zero point of 28.028 ± and ourown previous measurement (28.038 ± daophot under IRAF because of the fairly crowdednature of this region (Fig. 1) and the faintness of our targets. Wechoose an aperture equal to 3 × the full-width-at-half-maximumand checked that our targets were all well subtracted without resid-uals by our PSF analysis. We corrected the instrumental magni-tude for the z -band zero point and the airmass. We did not takeinto account possible effects due to colour terms. We list in Ta-ble 2 the final magnitudes of the five T–type candidates in USco.We note that we quote the mean value of the magnitudes whentwo measurements were available (case of 16084573 − − − To measure the relative proper motions for all common pointsources, we cross-matched the catalogues from the five NTT point-ings with the full catalogue of the deep WFCAM survey (Lodieuet al. 2011) with a matching radius of two arcsec. We found about3200 sources to compare with the proper motions measured for thefive T–type candidates. We list the proper motion in the right as-cension and declination as well as the total proper motion in Table2. We show their positions in proper motion reduced vector pointdiagrams in Fig. 2 where our T–type candidates are highlightedwith thick black triangles. All five candidates lie at least 2.5 σ fromthe mean absolute proper motion of the association estimated as( − −
25) mas/yr by Hipparcos (de Bruijne et al. 1997; de Zeeuwet al. 1999), arguing against their membership to the association.We compiled a list of known spectroscopic members of UScofrom Ardila et al. (2000), Mart´ın et al. (2004), Slesnick et al.(2006), Lodieu et al. (2006), Slesnick et al. (2008), Dawson et al.(2011), Lodieu et al. (2011), and Dawson et al. (2012) to cross-match with the catalogue of point sources common to the NTTfields-of-view and the deep WFCAM survey (Lodieu et al. 2011).Unfortunately, none of these known spectroscopic members lieswithin the NTT fields-of-view. This is not surprising considering (cid:239)
50 0 50 µ (cid:95) cos (cid:98) (mas/yr) (cid:239) µ (cid:98) ( m a s / y r) Figure 2.
Proper motion vector point diagrams for the five T–type candi-dates in USco marked with thick black triangles. The large circle has aradius of 12 mas/yr and is centered on the USco mean proper motion. Thesmall grey dots represent all point sources common to the deep WFCAMsurvey and the NTT fields. that the total area covered by the five NTT pointings is of the orderof 0.03 square degrees. Lodieu et al. (2006) and Lodieu et al. (2007)found between 0.1 and 0.5 member candidates in 0.03 square de-grees down to the depth of the UKIDSS GCS, depending on thelocation in the association.
Using our new photometry we have derived the J − H colours ofthe five USco candidates so that we can further probe their nature(Table 2). We find that these lie in the range 0.32–0.55 with anupper limit on the photometric errors of 0.18 mag, implying thatthese sources could be either T2–T4 dwarfs or M dwarfs due to thedegeneracy in the near-infrared colours (Hawley et al. 2002; Westet al. 2005; Hewett et al. 2006; Pinfield et al. 2008).To break this degeneracy, we determined their z − J coloursand find these to be in the range 1.63–1.83 ± > c (cid:13) , 1–, 1–
Using our new photometry we have derived the J − H colours ofthe five USco candidates so that we can further probe their nature(Table 2). We find that these lie in the range 0.32–0.55 with anupper limit on the photometric errors of 0.18 mag, implying thatthese sources could be either T2–T4 dwarfs or M dwarfs due to thedegeneracy in the near-infrared colours (Hawley et al. 2002; Westet al. 2005; Hewett et al. 2006; Pinfield et al. 2008).To break this degeneracy, we determined their z − J coloursand find these to be in the range 1.63–1.83 ± > c (cid:13) , 1–, 1– ?? roper motions of Upper Sco T–type candidates z (cid:60) J M L T
Figure 3. z − J colours of the five T–type candidates as a function of spec-tral type (red solid lines). The typical colours of M, L, and T dwarfs fromSloan (Hawley et al. 2002; West et al. 2008; Schmidt et al. 2010) are markedas asterisks. The M, L, and T regions are delineated by vertical dotted lines. Combining our new z and H photometry with our proper motionmeasurements, we conclude that the five candidates proposed byLodieu et al. (2011) as young, T–type candidates are not cool browndwarf members of the USco association. Hence, up to now, no T–type brown dwarf has been confirmed astrometrically and spectro-scopically in this region. Overall, there are no young T-types con-firmed spectroscopically in young star-forming regions, except theobject reported by Marsh et al. (2010) but questionned by Alves deOliveira et al. (2010). No astrometric confirmation is available andit will remain hard in ρ Ophiuchus due to the small mean motionof members of this region.We note the low success rate of wide-field surveys of youngpopulations involving methane filters. For example, all of our fivecandidates are rejected after obtaining second epoch imaging andadditional photometry. Burgess et al. (2009) reported three T–typecandidates but later rejected two of them using optical imaging.Similarly, Spezzi et al. (2012) reported four T–type candidates froma methane imaging combined with
JHK photometry and rejectedtwo of them (the third may be a non member too) from their posi-tions in various colour-colour and colour-magnitude diagrams. Webelieve it is essential that additional photometry (e.g. deep optical i and z ), spectroscopy or astrometry is obtained for the 22 candidateT-type members of ρ Oph identified by Haisch et al. (2010) so thattheir nature can be more rigorously examined.So, after eliminating the five T-type candidates here, we haveonly one photometric candidate left (UGCS J16064645 − J fainterthan ∼
19 mag. We reach a 5 σ limit of 21 mag, similar to the deepVISTA survey of 0.78 square degrees in σ Orionis of Pe˜na Ram´ırezet al. (2012) where three T–type candidates were identified, al-though proper motions indicate that two of them are likely to benon-members. Hence, our results are consistent with the main con-clusions of Pe˜na Ram´ırez et al. (2012) that we may see a turnoverof the mass function below 10–4 Jupiter masses (depending on theisochrones (NextGen, DUSTY, BT-Settl) used and the age electedfor USco (5 or 10 Myr) Baraffe et al. 1998; Chabrier et al. 2000;Allard et al. 2012) unless young T–type brown dwarfs are fainterthan predicted by state-of-the-art models.The next step in our quest for the bottom of the stel-lar/substellar initial mass function in USco is to obtain deeper andwider imaging using a combination of
Z, Y, J passbands where theUSco sequence can be clearly de-lineated from the general fieldpopulation (Lodieu et al. 2007). We have targeted over 10 squaredegrees in USco with the largest infrared camera in the world,VIRCAM (Dalton et al. 2006), installed on VISTA to address thisfundamental question regarding the fragmentation limit (Low &Lynden-Bell 1976; Rees 1976). Our results will be presented in aforthcoming paper.
ACKNOWLEDGMENTS
NL was funded by the Ram´on y Cajal fellowship number 08-303-01-02 and the national program AYA2010-19136 funded by theSpanish ministry of Economy and Competitiveness (MINECO).We thank Nigel Hambly for his advice on proper motion measure-ment.This work is based on observations made with the ESO NewTechnology telescope at the La Silla Paranal Observatory underprogramme ID 089.C-0854(A) in visitor mode, and with the GranTelescopio Canarias (GTC), operated on the island of La Palma inthe Spanish Observatorio del Roque de los Muchachos of the Insti-tuto de Astrof´ısica de Canarias.
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