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The Solar Neighborhood XXVII: Discovery of New Proper Motion Stars with mu>0.18"/yr in the Southern Sky with 16.5>R_59F>18.0

Here we present 1584 new southern proper motion systems with mu>0.18"/yr and 16.5>R_59F>18.0. This search complements the six previous SuperCOSMOS-RECONS (SCR) proper motion searches of the southern sky for stars within the same proper motion range, but with R_59F<16.5. As in previous papers, we present distance estimates for these systems and find that three systems are estimated to be within 25 pc, including one, SCR 1546-5534, possibly within the RECONS 10 pc horizon at 6.7 pc, making it the second nearest discovery of the searches. We find 97 white dwarf candidates with distance estimates between 10 and 120 pc, as well as 557 cool subdwarf candidates. The subdwarfs found in this paper make up nearly half of the subdwarf systems reported from our SCR searches, and are significantly redder than those discovered thus far. The SCR searches have now found 155 red dwarfs estimated to be within 25 pc, including 10 within 10 pc. In addition, 143 white dwarf candidates and 1155 cool subdwarf candidates have been discovered. The 1584 systems reported here augment the sample of 4724 systems previously discovered in our SCR searches, and imply that additional systems fainter than R_59F = 18.0 are yet to be discovered.

The SCR searches have now found 155 red dwarfs estimated to be within 25 pc, including 10 within 10 pc. In addition, 143 white dwarf candidates and 1155 cool subdwarf candidates have been discovered. The 1584 systems reported here augment the sample of 4724 systems previously discovered in our SCR searches, and imply that additional systems fainter than R 59F = 18.0 are yet to be discovered.

Introduction
The Research Consortium On Nearby Stars (RECONS) 1 has completed surveying the southern sky for proper motion objects with R 59F (hereafter R) brighter than 16.5 mag using a specialized trawl of the individual plate detections comprising the SuperCOSMOS Sky Survey database, with new discoveries dubbed SCR (SuperCOSMOS-RECONS). These results have been detailed in six papers in the The Solar Neighborhood (TSN) series: Hambly et al. (2004), Henry et al. (2004), Subasavage et al. (2005a,b), Finch et al. (2007), and Boyd et al. (2011) (TSN VIII, X, XII, XV, XVIII, and XXV, respectively). This seventh proper motion search paper complements the previous six by searching the entire southern sky for objects with the same lower proper motion cutoff of µ ≥ 0. ′′ 18 yr −1 as in previous papers, while examining objects fainter than the previous R cutoff. In this paper we stretch our search from R = 16.5 to 18.0 to reveal stars intrinsically fainter than found in previous searches, as well as stars similar to previous discoveries but at larger distances. In previous searches, it was found that several stars, including SCR 1845-6357AB, the 23rd nearest system (as of 1 January 2011), had R between 16.0 and 16.5, very near the search cutoff. It is natural, then, to assume that there may be more very nearby systems just beyond this cutoff that have heretofore gone unnoticed.
Despite far outnumbering their brighter counterparts, intrinsically faint nearby stars are underrepresented in the census of nearby stars, as evidenced by the discovery of many nearby red dwarfs (Henry et al. 2006) and white dwarfs (Subasavage et al. 2009). In particular, of the 256 systems with accurate parallaxes placing them within 10 pc, 174 (68%) have primaries of spectral type M. This search has been designed specifically to detect such faint objects. This effort is the latest in a long line of searches that revealed faint nearby stars, such as the classic sky surveys by Giclas et al. (1971Giclas et al. ( , 1978 and Luyten (1979. Many more surveys have since been done utilizing modern technology but fundamentally similar techniques and are discussed in TSN XXV and references therein. In this paper, we present photometric distance estimates for the red dwarf systems found during this search. Using the relations of Hambly et al. (2004), we estimate three red dwarfs to be within 25 pc, including one estimated to be at 6.7 pc. Using reduced proper motion diagrams, we also identify 97 white dwarf candidates, seven of which may be within 25 pc (see § 5.2). Particularly noteworthy for this survey, we report 557 new cool subdwarf candidates, nearly doubling the total identified during the SCR searches. The 1584 systems and 1608 objects reported in this paper bring the total number of SCR systems to 6308, consisting of 6650 objects 2 . Identifying comprehensive samples of proper motion objects now will prepare us for large-scale astrometric surveys such as LSST and Gaia, as well as provide precursor investigation opportunities. Ultimately, these new groups of intrinsically faint objects will help develop accurate models of the Galaxy as a whole.

Search Criteria and Methodology
This search has been performed with methods similar to those discussed in previous TSN papers. In brief, we make use of SuperCOSMOS scans of at least three Schmidt survey photographic plates taken of each field. The photographic plates scanned into the SuperCOSMOS database are 6 • × 6 • with a 0.5 • overlap on each side, giving ∼25 • square of unique sky coverage per field (in order to streamline computations of astrometric and photometric data, the overlap regions were not used). For this search, we have examined 893 of 894 fields, with one omitted because of an insufficient epoch spread and a location too near the Galactic plane. Thus, this search encompasses the entire southern sky save for one region 25 • square.
For this search, we require the R magnitude to be between 16.5 and 18.0 mag, with the brighter limit corresponding to the faint limit of previous searches, and the fainter limit being chosen to reveal late M dwarfs out to 10 pc. At 25 pc, we estimate that red dwarfs with spectral types earlier than M7.5V will have apparent R magnitudes greater than 18.0, with this search specifically targeting M5.5V to M7.5V.
We have used similar plate ellipticity requirements as TSN XXV: any object with two or more ellipticities greater than 0.35 is excluded. This requirement was shown in TSN XXV to eliminate significant amounts of false detections while removing no real proper motion objects in a trial sample of ∼200 stars. The ellipticity checks, along with the three-plate detection requirement preserves high quality candidates that went undetected on a single plate. SCR 1845-6357AB ), for example, was undetected on the B plate. Finally, we exclude objects with R ESO and R magnitudes differing by more than 1.0 mag in an effort to eliminate variable giant stars. We find 7626 initial candidates on these plates.
This list of candidates was then checked using Aladin by blinking the B J , R, and I IV N (hereafter BRI) plates with a 5 ′ radius to confirm that the objects are truly moving. 2MASS JHK s (hereafter JHK) magnitudes, coordinates, and observation epochs were extracted for confirmed proper motion objects that were then checked against SIMBAD and other proper motion surveys [e.g., NLTT -Luyten (1980), LEHPM -Pokorny et al. (2003), SIPS -Deacon et al. (2005), UPM -Finch et al. (2010)] using VizieR to determine which objects are new SCR discoveries. In both Vizier and SIMBAD, a region 90 ′′ in radius was used to match candidate objects to published objects, in accordance with the findings of Bakos et al. (2002), who found that coordinates of stars in the Luyten half-second (LHS) catalog were usually accurate to within ∼90 ′′ (see their Figure 2). In the blinking process, several common proper motion (CPM) candidates were noted that were not extracted in the initial trawl, but were found by eye (see § 5.5).

Comparison to Previous Searches
Here we compare the discovery statistics of this effort to previous SCR searches. We continue to use the terminology used in TSN XXV to describe the various proper motion bins called MOTION, SLOWMO, and MINIMO, which contain systems with µ ≥ 1. ′′ 00 yr −1 , 1. ′′ 00 yr −1 > µ ≥ 0. ′′ 50 yr −1 , and 0. ′′ 50 yr −1 > µ ≥ 0. ′′ 18 yr −1 , respectively. The two lower cutoffs were chosen to match those of the Luyten Half Second (LHS) and Luyten Two Tenths (LTT) catalogs, respectively. The results are summarized in Table 1, where the first number in each column is from previous searches while the second is from this effort. We note that significant amounts of garbage (i.e., false detections) were found in this search. These candidates, as in previous searches in high proper motion regimes, are primarily detections with erroneously high proper motion data. Matching high proper motion objects across epochs is more prone to mismatches than matching slow-moving or non-moving objects, particularly at faint magnitudes, hence the amount of garbage at higher proper motions is significantly larger.
We also update the hit rates -the sums of new, known, and duplicate objects divided by the total starting sample of all candidate objects -for each proper motion bin. Known objects are defined as those found to be previously discovered by other groups as outlined in § 2, whereas duplicates are objects for which more than one hit was returned from the SuperCOSMOS database. Garbage objects are those that are returned from the search but were found to be erroneous point source matches that were not verified to be proper motion objects. The new hit rates for the SCR sample are 7.1%, 81.0%, and 84.6% for the MOTION, SLOWMO, and MINIMO samples, respectively. The MOTION hit rate is slightly lower than the most recent update in TSN XXV due to the substantial amounts of garbage found in this search, while hit rates for the other two samples are somewhat higher, due to search criteria excluding excessive garbage in those proper motion bins.
In the NLTT catalog, there are 4356 objects that meet the criteria of this search. Of these, we recover 2782, or 63%, which is somewhat lower than the 71% recovered during the search in TSN XXV. Our cumulative total of 6308 systems for our entire southern hemisphere search represents an increase of 24.6% over the 25616 entries in the southern NLTT catalog.

Data
In Table 2, we list the first five of the 1608 newly discovered objects. All 1608 are listed in the electronic version of Astronomical Journal. We provide SCR names, J2000.0 coordinates, SuperCOSMOS proper motions and position angles, BRIJHK magnitudes, the (R − J) color, and a distance estimate. We report a total of 1584 systems, which is less than the number of objects because (a) some systems have more than one SCR object and (b) systems including both a known and SCR object are not included in the number of systems. We list red dwarfs within 25 pc, white dwarfs, and cool subdwarfs in Tables 3, 4, and 5, respectively. The white dwarfs and subdwarfs have been selected according to the criteria outlined in § 5.2. Multiple systems are listen in Table 6 and discussed in § 5.5.
TSN XVIII and XXV, as well as Finch et al. (2010), compare SuperCOSMOS data to data from various catalogs such as Hipparcos, Tycho, PPMX, and PPMXL. The results indicate that the SCR proper motions are consistent to other sources to ∼0. ′′ 02 yr −1 in both RA and DEC. The details of those comparisons can be found in the TSN XVIII and XXV.
The distance estimates in Tables 2, 3, 5, and 6 have been derived using the suite of 11 plate relations in Hambly et al. (2004), which utilizes the six photometry values, -BRI plate magnitudes from SuperCOSMOS and JHK from 2MASS -associated with each object. That paper and TSN XXV describe errors associated with the relations in more detail. The plate photometry errors are ∼0.3 mag, while 2MASS data errors are typically less than ∼0.03 mag. The relations fail for blue stars, as well as white dwarfs and most subdwarfs. For this reason, we list such objects' distances in brackets in the Tables 2, 3, 5, and 6. White dwarfs have more accurate distances listed in the notes of Tables 2, 4, and 6.

Analysis
In this search we have found a few red dwarfs estimated to be within 25 pc, as well as many white dwarf and cool subdwarf candidates. The numbers in these categories, however, are significantly different from previous papers. We find only three red dwarfs within 25 pc, listed in Table 3. We estimate the nearest, SCR 1546-5534, to be at 6.7 pc. If this estimate holds true, it will become the second nearest object found by the SCR searches, after SCR 1845-6357AB at 3.5 pc (Henry et al. (2006)). We find a total of 97 white dwarfs using two reduced proper motion diagrams (see § 5.2), tripling the total number of candidates from previous SCR searches, during which we found 46. Seven of the candidates from this search are estimated to be within 25 pc. Of particular note, we find 557 new cool subdwarf candidates, many of which are redder than those previously found. This total nearly doubles the 598 found in all previous SCR searches and comprises over one third of the sample in this paper. The remaining objects are likely to be early-to mid-type M dwarfs beyond 25 pc. Figure 1 shows the color-magnitude diagram for all SCR discoveries to date. Discoveries from the previous six searches are marked with small circles, while those presented here are marked with large circles. This search reveals sources that are redder than previous searches. Of note is the point at (R − J) = 7.01, which is SCR 1546-5534, the nearest and reddest system discovered by this search. The bluest objects are all white dwarf candidates, with the bluest being SCR 0221-0143, with (R − J) = 0.12.

Color-Magnitude Diagram
While not as easily discerned as in previous papers, the white dwarf and subdwarf regions are visible, with the white dwarfs extending to the left of (R − J) = 1.0 and the subdwarfs found between (R − J) = 1.0 and (R − J) ∼ 2.5. Further discussion of those systems can be found in the next three sections.

Reduced Proper Motion Diagram
Figures 2 and 3 show reduced proper motion (RPM) diagrams for SCR objects found during the present survey. The RPM diagram is a powerful tool for estimating the luminosity class of a star. It is similar in nature to the H-R diagram except that the distance is replaced by the star's proper motion, relying on the inverse statistical relationship between proper motion, µ, and distance. While obviously not foolproof -for example, white dwarfs may mimic subdwarfs, and vice versa in the RPM diagram -the diagram allows for the rough classification of systems. The equation used here to determine the pseudo-absolute magnitude H R 59F plotted on the vertical axis of Figure 2 is The dashed line used to separate white dwarf and subdwarf candidates in Figure 2 is the same as in RPM diagrams in previous papers. The solid lines, as in TSN 25, denote the cool subdwarf region. Cool subdwarfs are defined as having (R − J) > 1.0 and are within four magnitudes of the white dwarf line. These definitions of both the white dwarf and subdwarf regions are, of course, arbitrary, but have proven to be reliable in selecting high quality candidates. We find a total of 42 white dwarf candidates and 557 cool subdwarf candidates in this plot.
New to our SCR searches, we have included a second RPM diagram in addition to the (R − J) version. To reveal white dwarfs too faint to be in 2MASS, we use the (B − R) color because many WDs will not show up in 2MASS given our search limits of 16.5 < R ≤ 18.0 and the typical (R − J) < 1.0 color of WDs. In order to calculate H B we use the equation above, substituting B for R. We use the same WD-subdwarf cutoff line as Oppenheimer et al. (2001) to identify an additional 55 white dwarf candidates. We do not select any more cool subdwarf candidates because subdwarfs and main sequence stars are indistinguishable in this RPM diagram. Further discussion of white dwarfs and subdwarfs can be found in Sections 5.3 and 5.4.

New White Dwarf Candidates
We find a total of 97 new white dwarf candidates using the two RPM diagrams, presented in Table 4 and divided into two categories based on the RPM diagram from which they were selected. We present the same data as Table 2 except in lieu of the inaccurate distance estimates generated assuming the objects are red dwarfs, we give distances based on the single color linear relation of Oppenheimer et al. (2001), assuming the objects are white dwarfs. We find seven WDs estimated to be within 25 pc, all from the (R − J) RPM diagram. Of these, five have either optical or IR colors that are redder than typical white dwarfs. Two, SCR 0058-6905 and SCR 0816-4641, do appear to be good nearby white dwarf candidates. Three of the seven 25 pc candidates presented lie within the 13 pc horizon within which Holberg et al. (2002) suggested the white dwarf sample is largely complete. All three have colors redder than typical WDs, suggesting they may in fact be subdwarfs contaminating the white dwarf region of the RPM diagram.
The faintness of the WDs selected via the (B − R) RPM diagram suggests they are all quite distant, and indeed the distance estimates according to the relation of Oppenheimer et al. (2001) show the nearest to be at 40 pc.
TSN XXV discusses the 46 previous WD candidate discoveries, while the current effort adds 97 objects to this sample. In total we estimate 17 to be within 25 pc, of which seven are from this paper. TSN XXV discusses parallax results from CTIOPI for four of these white dwarfs, confirming they are within 25 pc. Spectroscopic observations are underway for unconfirmed WD discoveries. Table 5 provides the first five of the 557 cool subdwarf candidates. Because of their faintness compared to their similarly-colored main sequence counterparts, the distance estimates reported are too large and are listed in brackets in Tables 2 and 5. Figure 4 shows histograms of subdwarfs found in previous SCR searches as well as this work. The top plot shows we find more subdwarfs in the bin from 0. ′′ 18 yr −1 ≤ µ ≤ 0. ′′ 20 yr −1 , but fewer subdwarfs with µ > 0. ′′ 30 yr −1 than previous works. The bottom plot shows we discover redder subdwarfs in this work. The mean (V − K) color 3 of subdwarfs discovered in previous efforts is 3.2, while it is 3.8 in this effort. The redness may be explained in part by these systems being more distant discoveries than those from previous papers, and hence being affected by interstellar reddening. In the previous searches, the reddest subdwarf candidate has (V − K) ∼ 5.21 (SCR 0020-2642B), while here we find one with (V − K) ∼ 5.74 (SCR 1522-0244), which is similar in color to LHS 377, an M7.0 subdwarf (Gizis (1997), (V − K) = 5.76). The HR diagrams in Gizis (1997) and Jao et al. (2008) show that there are very few cool subdwarfs redder than M5.0. In recent years a few L type subdwarfs have been identified (Burgasser et al. (2009) and references therein), but more are needed to to explore the hy-drogen burning limit that comprise the end of main sequence. This work has increased the number of cool subdwarf candidates, but future astrometric and spectroscopic observations are needed to confirm them.

Common Proper Motion Systems
We have found 24 potential common proper motion (CPM) systems, including one triple, listed in Table 6. Of these, 19 consist entirely of new discoveries, while the other five have at least one known component. In Table 6 we provide distance estimates for all components, proper motions, separations, and position angles for both proper motions and separations.
Systems were found by one of two methods. First, during the blinking process systems were noted as having similar proper motion and position angles. Second, an automated search for companions within a radius of 1200 ′′ , as well as ∆µ < 0. ′′ 025 yr −1 and ∆ PA < 10 • , was performed around each object in the search.
The database from which the initial sample was drawn is a version of the database optimized for high proper motion objects. For systems noticed by eye that were not in the initial search sample, we manually gathered SuperCOSMOS data from the SuperCOSMOS Sky Survey (SSS) website. For moving objects, the SSS online catalogs tend to be less reliable and less complete, even at relatively low proper motions, as discussed in TSN XXV. We divide the table into two sections, probable and possible systems. Probable systems have µ and PA within 0. ′′ 025 yr −1 and 10 • , respectively, as well as consistent distance estimates, while probable systems appear to be comoving on plate images but either lack data to confirm common proper motion or have data that suggest otherwise.
Figures 5 and 6 compare the proper motion sizes and position angles for components of multiple systems, respectively. Systems marked with solid points are those with both components retrieved during the initial automated search, and tend to have better agreement between these values, particularly for the proper motions. Systems for which at least one component had its proper motion and position angle data gathered manually from the SSS database are marked with open circles. Figure 7 shows the sky distribution of systems presented in this paper, with the curve representing the Galactic plane. As in TSN XXV, we note a string of new systems superimposed on background dust in the Galactic plane. We also note a lack of discoveries in the areas near the Galactic center not obscured by dust. When compared to Figure 7 in TSN XXV, the NLTT system distribution in the southern sky, we note a similarly high density of new systems in areas searched less thoroughly by Giclas and Luyten. In the area between declinations -15 • and -30 • , there are relatively few new systems. This is because this region of the sky has been searched particularly deeply, notably by Giclas and Luyten as well as the LEHPM survey. A similar pattern exists, but was not noted, in the distribution presented in Figure 6 of TSN XXV.

Comments on Individual Systems
Here we highlight a few of the 1584 systems reported from this portion of the SCR search. These represent extremes in proximity, color (both blue and red), and/or sky separation.
SCR 0058-6905 is the second nearest white dwarf candidate, estimated to be at 12.1 pc, and one of three within the 13 pc limit of completeness according to Holberg et al. (2002). Its nature as a white dwarf is somewhat questionable, however, as its (B − R) color is redder than most white dwarfs.
SCR 0128-7401 is the third nearest white dwarf candidate, estimated to be at 12.8 pc, and one of three within the 13 pc limit of completeness according to Holberg et al. (2002). Its optical and near IR colors, however, are redder than most white dwarfs, calling in to question its nature.
SCR 0221-0143 is the bluest system discovered in this search with (R − J) = 0.12. It is a white dwarf candidate at 46.5 pc.
SCR 0307-4654A/LEHPM 1-3098 is the most widely separated system in this search, with a separation of 1180. ′′ 3. Using the mean distance of the system, this gives a projected sky separation of nearly 200,000 AU. While this system may be a random alignment of two unrelated stars, if the two stars comprise a system, it would be among the most weakly bound and widest systems known.
SCR 1546-5534 is the nearest star revealed by this search. It is the second closest system revealed by the SCR searches, after SCR 1845-6357AB at 3.5 pc, as reported in Henry et al. (2006). With (R − J) = 7.01, it is also the reddest object revealed by the SCR searches. We note that this object only has eight plate relations, with distance estimates ranging from 3.1 to 15.7 pc, calling into question its estimate of 6.7 pc. Even so, it is worthy of follow up and is currently being observed as part of RECONS' ongoing parallax program at the CTIO 0.9m telescope.
SCR 2045-1411 is the nearest estimated white dwarf candidate at 10.7 pc, and one of three that lie within the 13 pc limit of completeness according to Holberg et al. (2002). It is, however, unlikely to be an actual WD. Its (B − I) and (J − K) colors are significantly redder than known white dwarfs, and unlike such known white dwarfs, it satisfies all 11 red dwarf plate relations. It is likely a cool subdwarf candidate contaminating the white dwarf area of the RPM diagram.

Discussion
This paper completes an SCR sweep of the southern sky to R = 18.0 for systems with µ ≥ 0. ′′ 18 yr −1 . In total, we have found 6308 new proper motion systems, of which 1584 are first reported in this paper, representing an increase of 33% over the previous count. A total of 155 red dwarfs estimated to be within 25 pc, including 10 within 10 pc, have been found, of which three and one originate from this paper, respectively. We have found an additional 97 white dwarf candidates, bringing the total to 143, and an additional 557 cool subdwarf candidates, bringing the total to 1155. This paper is unique for the sheer numbers of white dwarf and subdwarf candidates revealed, with total counts that have tripled and doubled, respectively. Given the nature of their selection, there is obviously some contamination of the samples, but regardless, this represents a substantial increase. Of the 97 white dwarfs presented here, seven are estimated to lie within 25 pc. The subdwarfs presented here are redder and cooler than those previously discovered, helping fill in a currently underrepresented sample of cool subdwarfs discovered by these searches.
As part of our Cerro Tololo Inter-American Observatory Parallax Investigation (CTIOPI), an astrometry program carried out at the CTIO 0.9m  , we aim to obtain accurate trigonometric parallaxes for systems estimated to be near the Sun from the categories of 25 pc objects, white dwarfs, and cool subdwarfs. While the sheer number of candidates for observation means we can only select the best gems, the SCR search efforts will provide a wealth of targets for particular attention in large scale astrometric efforts such as Gaia and LSST. Such observations will help us more map the solar neighborhood more accurately than ever before.

Acknowledgments
The RECONS effort is supported by the National Science Foundation through grant AST 09-08402. Funding for the SuperCOSMOS Sky Survey was provided by the UK Particle Physics and Astronomy Research Council. N.C.H. would like to thank colleagues in the Wide Field Astronomy Unit at Edinburgh for their work in making the SSS possible; particular thanks go to Mike Read, Sue Tritton, and Harvey MacGillivray. This research has made use of results from the SAO/NASA Astrophysics Data System Bibliographic Services, the SIMBAD and VizieR databases operated at CDS, Strasbourg, France, and the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center, funded by NASA and NSF.      17.08 · · · · · · · · · 55.5 a Estimate given using relation of Oppenheimer et al. (2001) b CPM system -see Table 6 c Candidates lack 2MASS data. Coordinates not J2000.0.    -Histograms of SCR subdwarf candidates this paper (filled) and previous TSN papers (outlined). The top plot shows quantity versus proper motion bin, while the bottom shows quantity versus color. There are notably fewer high proper motion discoveries in this paper, but those subdwarf candidates discovered tend to be redder than previous discoveries.