arXiv:1408.0073v1 [astro-ph.SR] 1 Aug 2014 od oetyC47L UK 7AL, CV4 Coventry Road, UK 7RH, Mai S3 Sheffield Chiang Muang, Suthep, Rd., Kaew Thailand 50200, Huay Bldg., riphanich h rudo nsaebcmspsil.Significant possible. becomes on space telescopes in largest or the ground even beyond the of well limit sizes imaging. diffraction angular standard the of in measurement as the telescope than Thus, the rather of , aperture the the to resolution distance the angular the and the between wavelength relations that ad- on dependent major fact mainly One the is achieved is limb. LO lunar of the vantage by background of occulted a analysis is when the source generated of pattern means diffraction by the resolution angular high Introduction 1. 3 2 1 ua cuttos(O r ehdt obtain to method a are (LO) Lunar eateto hsc,Uiest fWrik ibtHill Gibbet Warwick, of University Physics, of Department Sheffiel of University Astronomy, and Physics of Department 191 Thailand, of Institute Research Astronomical National is ua cutto eut rmte24mTa National Thai m 2.4 the from Results Lunar First esn erpr nteueo hsfclt orcr ihti high 10 record several to as facility fast this as of sampling use with the subarrays on report we season, camera EMCCD frame-transfer low-noise, ULTRASPEC the with ihatlsoeo hscas mn h eut,w discuss we results, the Among this class. to this of contribution telescope first a the with represent which several ujc headings: Subject no where area geographical a now. wo in occultation technique lunar occultation for d TNT lunar available at ULTRASPEC other of of performance context the discu the We in separation orbit. projected undetermined 707 and yet HR binary its the for of available measurement coverage a present also We companion. aes a e ihalc fuabgoscnrain iho With ( separation confirmation. angular unambiguous projected of the lack on a limits with stringent At met binary. far close suspected so a have as previously reported been has tr:idvda:H 7072 HR individual: stars: h eetyiagrtd24mTa ainlTlsoe(TNT Telescope National Thai m 2.4 inaugurated recently The .Richichi A. ehius ihaglrrslto cuttos–binar – occultations – resolution angular high techniques: 1 .Irawati P. , qipdwt ULTRASPEC with equipped 1 [email protected] .Soonthornthum B. , ABSTRACT Si- 2 d, z npriua,w aercre ua cuttosof occultations lunar recorded have we particular, In Hz. 1 < 0 ag hc ean nupse.I h oreo a of course analyze, dynamic the In and to sensitivity unsurpassed. easy remains of observe, which combination range to a quick offer edge are and an they maintain Con- that LO in however, 2005). stars, al. binary et used Richichi cerning often catalog, is results, CHARM2 its cur- (see in con- but complete time more above, but more suming areas interferometry, two long-baseline made first have rently the LO decades, in For contributions stars. small- binary of study separation the finally and emissions, extended and envelopes de- circumstellar the of characterization diameters, and angular tection stellar of measurement the res- ms. time 1 with order curves of light olutions the fringes, sample diffraction to the necessary measure is it and rare. detect are to observations order the In and repeat time-critical, to are events cannot opportunities the that sources that the will, at that chosen are be method the of limitations . ′′ t.W ocueb rvdn netmt of estimate an providing by conclude We ata. rao eerhmd rmSuhEs Asia South-East from made research of area 0)adbihns ( brightness and 003) tlattremjrgascnb civdb LO: by achieved be can goals major three least At sorpeiedtriaino h u ratio flux the of determination precise our ss epst eov hssa ysvrlauthors several by this resolve to tempts erslto mgn sn ml detector small using imaging resolution me 1 nacrt esrmn of measurement accurate an oprbersucswr vial until available were resources comparable ..Dhillon V.S. , tteedo t rtofiilobserving official first its of end the At . seupe,aogohrinstruments, other among equipped, is ) ,wihetnscnieal h time the considerably extends which 2, k hsfclt a ept xedthe extend to help can facility This rk. rosrainw r bet place to able are we observation ur e:gnrl–sas individual: stars: – general ies: 2 ..Marsh T.R. , ∆ m > )o putative a of 5) α Telescope n,which Cnc, 3 α n – Cnc long program of occultations with the ISAAC instru- capabilities for LO work. ment at the ESO VLT, Richichi et al. (2014, and refer- In order to increase the time sampling, ULTRA- ences therein) have recorded over a thousand occulta- SPEC offers the possibility to read out only parts of tion events with an ≈ 8% detection rate of mostly new the detector. One of its features is the 2k×1k EMCCD binary or multiple systems, the majority of which are chip, half of which is used for imaging and half for out of reach of other techniques. The ISAAC instru- fast frame transfer. One or several subarray windows ment has now been decommissioned, and only few ob- can be defined and read out simultaneously, and this is servatories around the world remain suitably equipped optimal to reach rates up to about 10 Hz. For LO mea- to observe LO. surements, however, faster rates are needed. For this, In this paper we first introduce the Thai National the so-called drift-mode of the related instrument UL- Telescope (TNT), in conjunction with the ULTRA- TRACAM Dhillon et al. (2007) has been specifically SPEC fast camera, as a new facility capable of record- adapted. In summary, two small windows can be se- ing high quality LO light curves in a geographical area lected along the same detector rows. Although their where this technique was so far precluded. We then position is freely selectable, the most efficient config- present the first LO results obtained at the TNT, and uration is to place them close to the lower edge of the discuss in detail two of them: α Cnc, a claimed bi- detector. A mask is then remotely positioned to cover nary star for which we can put stringent upper limits the rest of the detector area. As the selected windows on the separation and flux of the putative companion; are pushed to the frame transfer area, the rows immedi- and HR 7072, a known binary star for which the nu- ately above are shifted down and are ready for the next merous measurements available until now seem to be integration. Other features that we do not describe in not always in agreement and for which we provide a detail here are the possibility of on-chip rebinning and new accurate flux and separation determination. three levels of detector read-out speed. We have suc- cessfully tested the drift mode up to rates of 450Hz. 2. A New Facility for High-Time Resolution As- Thanks to a large memory and to fast fiber links be- tronomy tween the instrument electronics, the control computer and the data reduction machine, it is possible to obtain The Thai National Observatory, which includes also uninterrupted data sequences of any desired duration the flagship 2.4m Thai National Telescope (TNT), is and to examine the data almost in real time. Finally, located on one of the highest ridges of Doi Inthanon, we mention that a dedicated GPS is used to stamp each the tallest peak in Thailand. At 2457m elevation, the frame with its mid-exposure time. site has observing conditions which compare favor- ably with those of most other locations in the region 3. Observations and Data Analysis in terms of seeing and photometric conditions. It has a dry season which runs approximately from Novem- The first LO observations recorded from TNT with ber to April, while the rest of the is largely lost ULTRASPEC are listed in Table 1, in chronological to observations due to high humidity and rainy condi- order. D and R refer to disappearances and reap- tions. The TNT was erected in 2012 andinauguratedin pearances, respectively. The magnitudes and spectra January 2013. It is a Ritchey-Chrètien with two Nas- are quoted from Simbad. Concerning the filters, we myth foci, one of them being equipped with a multi- adopted mainly narrow-band ones in the attempt to instrument port which mounts permanently also the reduce the lunar background. The latter is strongly ULTRASPEC instrument. Although a similarly named wavelength dependent, and also diffraction is inher- instrument based on the same detector has been used ently chromatic. Thus, the red part of the spectrum before, e.g. at the ESO New Technology Telescope is usually better suited for LO observations. In the in Chile (Dhillon et al. 2008; Ives et al. 2008), the one table, HαN and HαB differ slightly in central wave- described here is novel in several aspects: in particular length (6564 and 6554 Å, respectively), and in FWHM the optics have been specifically developed for TNT, (54 and 94 Å, respectively). We also employed a stan- and specific data acquisition modes have been devel- dard SDSS z′ filter. The columns Sub and Bin list, re- oped. ULTRASPEC at the TNT had its first light in spectively, the size of the detector subarray adopted in November 2013 and is described in detail elsewhere the drift-mode and the on-chip rebinning - so that Sub (Dhillon et al. 2014). Here, we summarize only its 32x32 and Bin 2x2 would effectively result in a 16x16

2 TABLE 1 SUMMARY OF OBSERVED EVENTS

Date Time Type Source V Sp Filter Sub Bin τ ∆T S/N Notes (UT) (mag) (pixels) (ms)

11-Jan-14 17:12 D SAO 93721 5.9 F4V z′ 16x16 2x2 6.6 6.8 34 Unresolved 11-Feb-14 16:10 D SAO 96543 7.5 F5V HαB 16x16 no 11.8 12.1 10 Unresolved 12-Mar-14 18:26 D SAO 97913 6.3 K0III HαB 16x16 2x2 6.6 6.8 45 Unresolved 09-Apr-14 13:20 D α Cnc 4.3 A5m HαN 16x16 2x2 6.6 6.8 104 Not Binary 20-Apr-14 21:44 R HR 7072 6.5 A1V+K1III HαB 32x32 2x2 12.3 12.8 18 Binary

output. The frame integration time and the sampling and 1.5mas for SAO 93721 and SAO 97913, respec- time between frames are denoted by τ and ∆T in the tively. These are consistent with the values expected table. This latter is the average value across all frames. from their spectral types and distances. The S/N of the In reality, there are small variations (always < 1%) in SAO 96543 light curve was insufficient for an upper the time differences between subsequent frames. In limit determination. There were no previous literature our data analysis, the actual individual time stamps are reports on possible binarity for two of these sources, used. S/N is the signal-to-noise ratio, measured as the while SAO 93721 is listed in the Washington Double unocculted stellar signal divided by the rms of the fit Star Catalog. It has a faint, distant companion at about residuals. 170′′ which is not included in our observation, and is The raw binary output from ULTRASPEC is then also listed as a spectroscopic binary which we could converted to FITS cubes, with the first two dimensions not resolve. In the following, we concentrate on the set by the combination of Sub and Bin, and thousands remaining two stars in our list. of frames long. From this point, our data analysis pro- Although these first measurements are few, Table 1 cedure follows closely the methods already described can be used for some initial estimate of the perfor- in our previous papers, see e.g. Richichi et al. (2014) mance of LO with ULTRASPEC at the TNT. Using and references therein. In summary, we adopt a mask S/N=1 as a detection limit, the sensitivity achieved extraction that allows us to reject unnecessary signal in the 5 light curves ranged from 9.3 to 10.4mag, (and related noise) from the background, and end up with a dynamic range that in the best case was 5mag. with fast photometry sequences which are further re- The ability to resolve close companions was tested by stricted to very few seconds around the events. means of simulations on the data of SAO 93721 and We use a model-independent maximum-likelihood α Cnc. We conclude that hypothetical companions (CAL, Richichi 1989) method to estimate the bright- with 1:1 flux ratio could have been detected as close ness profile of the source, for example to detect possi- as 2 mas. ble multiple components in the light curve. A model- dependent least-squares method, whose convergence 4.1. Alpha Cancri 2 in χ is driven by noise components derived from the We obtained a good quality (S/N=104) light curve unocculted and totally occulted portions of the light of the bright star α Cnc (HR 3572, Acubens), shown in curve, is used to derive precise values and errors of pa- Fig. 1, along the position angle (PA) 77◦. The diffrac- rameters such as intensities and separation in a binary tion fringe pattern is well resolved thanks to the use of model (Richichi et al. 1996). This reference also de- a narrow band filter and also to a contact angle of 40◦ scribes additional features used in our fits such as the that contributed to slow down the apparent fringe mo- modelling of low-frequency scintillation. tion. We could thus accurately measure the local limb slope, found to be close to 0◦, obtaining in turn a re- 4. Results liable conversion from time to angular scale. The star is known to have a companion forming the ADS 7115 The first three sources in Table 1 were found to be pair; this is however about 10′′ away and several mag- point-like, with upper limits on the angular size of 2.4 nitudes fainter, and we do not concern ourselves with

3 it here. significant separation, of about 5mag in the red part of More interestingly, α Cnc has been claimed as a the spectrum. We mention also two light curves α Cnc, close double, mainly from previous LO observations the disappearance and reappearance of the same event, but also from the analysis of Hipparcos data. These are observed with a 30-inch telescope in a 5214Å inter- listed in the Fourth Catalog of Interferometric Mea- ference filter by Africano et al. (1978). The authors surements of Binary Stars (by Hartkopf, Mason and also found no evidence of binarity, but given the small Wycoff, available online1; INT4 hereafter). The two telescope size scintillation was significant and the con- claims from previous occultations come from observa- straint on a putative companion was weaker than that tions by amateur astronomers with small telescopes, from our data. and are not well documented. They claim a binary The other indication of binarity in α Cnc comes with equal components separated by 50mas in one from Hipparcos, which detected an acceleration in the case (with no PA listed), or 4.2mas along PA=113◦ , interpreted as due to a companion. in the other. We note that the ability to measure such However no orbital solution could be derived and the a small separation by LO usually demands a rigorous nature of the companion (separation and mass, hence treatment of high quality data. The Bright Star Cat- brightness) remains undetermined, and likely unre- alogue mentions in a note that α Cnc is a LO binary lated to the putative companion claimed by previous with 0.′′1 separation, but without a reference. Several LO reports. Frankowski et al. (2007), after examining speckle measurements with the SOAR 4.1m telescope several radial velocities catalogs, found no evidence of (Tokovinin et al. 2010; Hartkopf et al. 2012) have not α Cnc being a spectroscopic binary. detected the companion, with upper limits ≤ 0.′′15 on the angular separation and 4 to 6mag on the flux dif- 4.2. HR7072 ference. In fact, for ∆m≤ 1 mag the separation limit is Our reappearance light curve of HR 7072 (Kui 25mas (Tokovinin, priv. comm.). 88AB, HIP 92301) is shown in Fig. 2 and it clearly re- Our data appear to be the first LO light curve veals a companion. We find a flux ratio of 3.38±0.01, recorded of this bright star with professional equip- and a projected separation of 90.7±0.2mas along ment at a medium-sized telescope. We can reliably PA=238◦. In order to successfully record the reap- exclude two components of similar brightness, to pro- pearance event, we opted for a larger subarray which jected separations as small as 3mas (see Fig. 1). In resulted in a relatively slow sampling. Thus, we are fact, an analysis by a method developed for unresolved not able to determine independently the speed of the sources (see Richichi et al. 1996, 2012) results in a fringe pattern, and in turn we have an uncertainty on limit of 1.35mas on the angular size of the star. This the the local limb slope and correspondingly on the is consistent with a stellar dimension roughly compa- PA and projected separation. However, an analysis of rable to solar and a distance of 53pc as derived by the lunar limb at the given point of contact and libra- Hipparcos. For a simple comparison, we assume that tion angles was made using the altimetry data from the the 50mas binary separation previously claimed was Kaguya probe (Herald, priv. comm.), and no signifi- at a randomposition angle, and we consider an average cant local limb slope was found. projection factor 2/π. The resulting estimated 79 mas HR 7072 is a known sub-arcsecond binary, with true separation, against our upper limit of 3 mas, would over 30 previous astrometric and LO determinations, indicate a likelihood of 1−(acos(3/79)/90◦) ≈ 2% that starting from the initial discovery by Wilson (1950) our lack of detection was due to projection effects. and extending over 54 . A list can be found We note that the previously claimed binary separation in INT4, and additionally we mention Muller (1958). would imply a period of very few years, and therefore Our result extends the time coverage to almost 65 any link between the geometry during our measure- years. An orbital solution has not yet been established, ment in 2014 and the previous ones (as much as 30 and in fact the interpretation of the available data is not years earlier) would not be preserved. The S/N of our straightforward. light curve can also be used to assess an upper limit in flux on the presence of a companion with a more The left panel of Fig. 3 shows the RA, Dec position of the companion. For the LO results, which only pro- 1http://www.usno.navy.mil/USNO/astrometry/optical-IR- vide a projected separation, we have plotted the lines prod/wds/int4 that represent the loci of equivalent position. At a

4 200 a) b) 0 + 180 c) 160 PA [deg]

-200 600 140 Dec [mas]

400 -400

200 Separation [mas] +400 +200 0 1950 1970 1990 2010 RA [mas] Year Fig. 3.— Left: astrometric (circles) and LO (lines) determinations of the position of the companion of HR 7072 with respect to the primary (cross). The LO are a) Africano et al. (1978), b) Edwards et al. (1980), c) our measurement. Right: the same data, converted to PA (top) and separation (bottom) and plotted as a function of time. In this plot, the crossed symbols represent estimated LO positions, extrapolated from the lines in the left panel. The open symbols in both plots are discussed in the text. Errors are mostly not available. A color version of this figure is available online.

120

100 a) 126

80 b) Intensity 125

60 124 Intensity 2 x4 0 123 -2 Residuals 1000 1200 1400 0.3

Relative Time [ms] 0 -0.3 Fig. 1.— Top panel: occultation data (dots) for α Cnc, Residuals 4000 4400 4800 and best fit by a point-like source (solid line). The fit Relative Time [ms] residuals are shown in the lower panel, enlarged for clarity. The curves labelled a) and b) are models for a Fig. 2.— The light curve data for HR 7072 (dots) and binary star with equal components and projected sepa- the best fit by a binary star model (solid line). The fit rations of 3 and 10mas, respectively. They are shown, residuals are shown in the lower panel. shifted by arbitrary vertical offsets for clarity, to prove the inconsistency of the data with such scenarios. A color version of this figure is available online.

5 first glance, it can be noticed that the positions tend that the two stars have similar brightness in the blue, to cluster around the [+150,−400]mas region, with the but that at longer wavelengths the K0 component is the notable exception of the points on the negative RA primary. This is consistent with Jaschek et al. (1991), half-plane which are all prior to 1960. These were who found a 12 µm excess for this star and mentioned visual determinations, while later measurements were binarity as a possible explanation. obtained by speckle interferometry, The right panel of Fig. 3 puts the measurements, expressed now as PA 5. Conclusions and separation, along a time sequence. It can be seen that both quantities follow an almost linear evolution We reported the first LO observations from the with time, however with the noticeable exception of Thai National Telescope equipped with the ULTRA- two points. These are marked as outlined, rather than SPEC instrument. Using the specifically developed solid, symbols, on both panels of Fig. 3. The first one drift-mode, time sampling of few milliseconds can be is the discovery measurement by Wilson (1950), the achieved. In the first observing season of this new fa- second one is the latest measurement, prior to ours, cility, we have recorded 5 LO light curves and we have obtained by Mason et al. (2004). The former was ob- discussed in detail two of them. tained by visual interferometry, the latter by speckle, In the case of α Cnc a duplicity has been previously but both at relatively smaller telescopes than the major- claimed, although not convincingly demonstrated. We ity of the other determinations. We remark that most do not detect the companion, with an upper limit of of the literature values do not have an associated error, 3mas in projected separation and 5mag in flux ratio. so that it is difficult to verify quantitatively possible In the case of HR 7072, our measurementgoes to com- discrepancies. plement a set of over 30 previous determinations, ex- In the right panel of Fig. 3 we have made an at- tending the time coverage from 54 to 65 years. We tempt to estimate PA and separation for the two pre- show that the data are still ambiguous, pointing to the 2 vious LO measurements and our own, based also on a detection of an orbital arch and a few 10 y period, or visual fit to the rest of the points in the left panel. It alternatively to a much wider and longer orbit. Obser- can be seen that in this view the apparent discrepancy vations by adaptive optics would quickly confirm one of the Wilson (1950) measurementcould in fact be rec- of the two scenarios. The flux ratios are consistent with onciled given a reasonable error, as expected from the an early A and an early K spectral types, with similar method. The Mason et al. (2004) measurement is crit- brightness in the blue but with the K type component ical, in that taken at face value it points to a significant dominating at longer wavelengths. turn in the orbit. Accordingly, we provide two PA-sep The TNT with ULTRASPEC in drift mode has suc- equivalents for our LO result: one fits a linear trend in cessfully demonstrated high performance LO results, time of PA and separation, consistent with a very long with a sensitivity estimated at I ≈ 10mag at S/N=1, an orbital period. The other follows the indication of the angular resolution close to 1mas, and a dynamic range Mason et al. (2004) measurement, and reinforces the of at least 5 mag. This facility is especially attractive in view of a closed orbit which would then seem to have consideration of the fact the LO events are observable a period of few 102 y. only along restricted groundtracks, and that no compa- Concerning the flux ratio, the available data span rable capabilities existed until now in South-East Asia. the range from 400 to 800nm, and are quite consistent in their trend indicating that significant variability of VSD and TRM acknowledge the support of the either componentcan probably be excluded, see Fig. 4. Royal Society and the Leverhulme Trust for the op- Our own determination is in close agreement with this eration of ULTRASPEC at the TNT. We are grateful trend. The conclusionis that one of the two stars is sig- to Dr. A. Tokovinin for useful discussions. The esti- nificantly bluer than the other. Andersen et al. (1985) mation of the lunar limb slope for the occultation of measured the of this system on two HR 7072 was provided by Dr. D. Herald. This re- dates only, separated by just one year. They did not search made use of the Simbad database, operated at detect any variations. They quote the spectrum as com- the CDS, Strasbourg, France. posed of A1 and K0. Houk & Smith-Moore (1988) reported A1V and K1III. This, together with the in- tegrated color of V − K =2.8mag, points to the fact

6 REFERENCES Tokovinin, A., Mason, B. D., Hartkopf, W. I. 2010, AJ, 139, 743 Africano, J. L., Evans, D. S., Fekel, F. C., Smith, B. W., Morgan, C. A. 1978, AJ, 83, 1100 Wilson, R. H., Jr. 1950, AJ 55, 153 Andersen, J., Nordstrom, B., Ardeberg, A., et al. 1985, A&ASS, 59, 15 Dhillon, V. S., Marsh, T. R., Stevenson, M. J., et al. 2007, MNRAS, 378, 825 Dhillon, V. S., Marsh, T. R., Copperwheat, C., et al. 2008, High Time Resolution Astrophysics: The Universe at Sub-Second Timescales, 984, 132 Dhillon et al, 2014, About to be submitted, more de- tails before proofs. Edwards, D. A., Evans, D. S., Fekel, F. C., Smith, B. W. 1980, AJ, 85, 478 Frankowski, A., Jancart, S., Jorissen, A. 2007, A&A, 464, 377 Hartkopf, W. I., Tokovinin,A., Mason, B. D. 2012, AJ, 143, 42 Houk, N., & Smith-Moore, M. 1988, Michigan Cat- alogue of Two-dimensional Spectral Types for the HD Stars. Volume 4, -26◦.0 to -12◦.0, University of Michigan, Ann Arbor, MI 48109- 1090, USA Ives, D., Bezawada, N., Dhillon, V., Marsh, T. 2008, SPIE Conference Series, 7021, 10 Jaschek, C., Jaschek, M., Egret, D., Andrillat, Y. 1991, A&A, 252, 229 Mason, B. D., Hartkopf, W. I., Wycoff, G. L., et al. 2004, AJ, 128, 3012 Muller, P. 1958, Journal des Observateurs, 41, 109 Richichi, A. 1989, A&A, 226, 366 Richichi, A., Baffa, C., Calamai, G., Lisi, F. 1996, AJ, 112, 2786 Richichi, A., Percheron, I., & Khristoforova, M. 2005, A&A, 431, 773 Richichi, A., Cusano, F., Fors, O., Moerchen, M. 2012, ApJSS, 203, 33

Richichi, A., Fors, O., Cusano, F., Ivanov, V. D. 2014, This 2-column preprint was prepared with the AAS LATEX macros AJ, 147, 57 v5.2.

7 2

1 Flux Ratio A/B [mag]

0 400 500 600 700 800 λ [nm]

Fig. 4.— The flux ratio of the two components of HR 7072, as a function of wavelength. Solid dots are previous results, the outlined circle is our measure- ment. Some errorbars are too small to be seen clearly on this scale.

8