LUNAR OCCULTATIONS OF 18 STELLAR SOURCES FROM THE 2.4 m THAI NATIONAL TELESCOPE

This star (HR 510, HIP 8198) is a relatively nearby late-type giant star (G8, 85 pc) which has been the subject of several investigations concerning its possible binary nature. It was included in a catalog of stars whose Tycho and Hipparcos data showed nonlinear behavior with time, indicating possible astrometric binaries (Makarov & Kaplan 2005). As such, it was also included as possible binary in a catalog of bright multiple systems by Eggleton & Tokovinin (2008). Frankowski et al. (2007) did not find evidence of changes in the radial velocity, using a statistical analysis that was however mostly sensitive to stars with parallax >20 mas. Hartkopf & McAlister (1984) obtained speckle observations at a 4 m telescope, finding it unresolved with an upper limit of 30 mas. Interestingly, no LO event of this star was ever reported until now.

We obtained a good quality (S/N = 61), first ever LO light curve of o Psc. Thanks to a relatively large contact angle, the efffective limb velocity was about three times slower than normal, leading to an increased sampling of the fringes which was crucial in assessing departures from a point source. The data are consistent with a resolved angular diameter and in addition with a secondary component 4.0 mag dimmer than the primary, with a separation of 12 mas projected along PA = 144°. Our first-time determination of the angular diameter, when combined with the Hipparcos distance, results in 19.8 R_⊙.

The companion, given the magnitude difference, is likely to be a main sequence star, although more details could be ascertained only with further observations at different wavelengths. Some constraint on the actual PA can be inferred from the upper limit on the separation by Hartkopf & McAlister (1984). Assuming that their speckle measurements were sensitive to a companion with such a brightness ratio, the PA would have to be in the—admittedly broad—range 78°–210°. Assuming a combined 1 M_⊙, a real separation between 1 and 2.6 AU (converted from our measurement and from the speckle upper limit), a circular orbit and no inclination effects, the minimum period would be between 1 and 4 years. Barring significant inclinations of the orbital plane on the sky, orbital motion might be detectable within relatively short periods of time, using i.e., adaptive optics at a very large telescope at visual wavelengths.

This star is a late-type giant (G8II/III) and a bright near-infrared source (IRC –20325, K = 2.1 mag). No angular diameter measurements are present in the literature. Evans & Edwards (1981) obtained a LO light curve, finding the source unresolved. Their data were recorded in the blue with a small telescope and were presumably noisy. Another previous LO light curve is listed in the CHARM2 catalog (Richichi et al. 2005), however the result was not published. Pasinetti Fracassini et al. (2001) estimated a diameter of 2.0 mas.

We recorded a high S/N reappearance light curve of HR 6196. The data are best fitted by a resolved source model of 1.85 mas diameter, which leads to a χ² almost 2.5 times lower than for a point source. We also revisited the old data set mentioned in CHARM2, which was obtained with an InSb photometer in the K band at the Calar Alto 1.2 m telescope on 1997 July 16. Unfortunately, those data are affected by significant scintillation and only lead to an upper limit between 2.7 to 3.4 mas, consistent with our result but not very constraining.

This cool giant (HR 3950, IRC +10224) has had a number of spectral classifications, with a general agreement around M2. It has featured in many publications especially in the near-IR, where it has often been used as a calibrator source due to its brightness (K = 0.5 mag) and minimal variability (e.g., Monnier et al. 2004; Woodruff et al. 2008; Montargès et al. 2014).

The angular diameter of π Leo has been repeatedly determined in the past, with results listed in the uniform ϕ_UD, limb-darkened ϕ_LD or fully darkened ϕ_FD disk hypothesis. Using the NPOI interferometer at various wavelengths with an average of 740 nm, Nordgren et al. (1999) reported ϕ_UD = 4.29 ± 0.05 mas, in very good agreement with our own determination. A large number of LO observations also exist, with varying levels of data quality and accuracy. Vilas & Lasker (1977) reported ϕ_LD = 5.9 mas in the R-band (no error), while Africano et al. (1978) reported ϕ_FD = 3.9 ± 1.0 mas in the blue. We note that in both cases a small sub-meter class telescope was used, so that scintillation may have played a role. A "smoothing procedure" was claimed as a possible cause for the first high value (Africano et al. 1978). Later, Ridgway et al. (1979) reported ϕ_UD = 4.58 ± 0.33 mas, while Baug & Chandrasekhar (2013) obtained ϕ_UD = 4.9 ± 0.5 mas, both in narrow K-band filters. Schmidtke & Africano (2011) reported ϕ_UD = 3.85 ± 0.15 mas, using a 1991 LO light curve obtained in H_α with a 1.3 m telescope. The weighted average of these LO results, neglecting the difference in wavelengths and limb-darkening, gives at first approximation ϕ = 4.3 ± 0.3 mas.

We obtained a light curve for π Leo with a quality $({\rm{S}}/{\rm{N}}=147)$ better than any previous others. As shown in Figure 1, the source is clearly resolved, and our result listed in Table 2 is consistent with all good-quality previous determinations, but with significantly improved accuracy.

This is a bright M0 giant (HR 4371), especially luminous in the near-IR (IRC +00203, K = 1.4 mag) and included in many publications, some of which covering high-angular resolution. Despite several attempts, the star had remained until now essentially unresolved. These include speckle observations at a 4 m telescope with a stated upper limit of 35 mas by McAlister et al. (1989) and by Mason (1996), and a three-channel LO light curve from a 1.5 m telescope by Meyer et al. (1995). Guhl & Stecklum (1991) obtained a LO light curve which was consistent with a companion at a projected separation of 45 mas and 0.8 mag fainter. However, the data were obtained with a 40 cm telescope in an urban environment and the authors themselves cautioned about this finding. Having a stable luminosity, the star has also been included in catalogs of spectrophotometric indirect diameter estimates. For example, Cohen et al. (1999) quote ϕ_UD = 2.90 ± 0.03 mas.

Our light curve for 75 Leo has a formally very high S/N of 266. In fact, the data were taken with a rather slow sampling (and correspondingly long integration time, hence the high S/N value), due to passing clouds that made the star temporarily disappear during acquisition and forced us to observe in blind mode with an unusually large sub-window. As a result, the diffraction fringes are smoothed and poorly sampled. Nevertheless, these factors can be properly included in the data analysis and the χ² clearly shows that the star is indeed resolved. Our resulting angular diameter value is ϕ_UD = 2.94 ± 0.03 mas, in excellent agreement with the estimate of Cohen et al. (1999). This represents the only direct determination for 75 Leo. Barring a very unfavorable combination of projection directions, the bright companion possibly claimed by (Guhl & Stecklum 1991, who did not mention a PA for their event) should have been seen in our data.

This bright star (HR 763, SAO 93022) has a spectral type F7V. Being main sequence, it is as expected relatively nearby: its Hipparcos parallax places it at 35 pc. 31 Ari was first discovered to be binary from a LO by Africano et al. (1978), with projected separation 21 mas along PA = 265.7. The magnitude difference was found to be small both in the blue and in the red, prompting the authors to speculate similar spectral types. For comparison with our measurement (shown in Figure 2), they quoted Δmag = 0.3 ± 0.1.

Following up on this, 31 Ari was detected as binary using speckle by McAlister & Hendry (1982), and later reported in numerous other publications. Both Balega & Balega (1988) and Mason (1997) computed orbital elements. They are however in considerable disagreement. Using the latest, and presumably more complete, of the two we find that both older (e.g., Africano et al. 1978; Hartkopf et al. 2000, epochs 1977.75 and 1984.07) and newer observations (e.g., Hartkopf et al. 2012; Horch et al. 2012, epochs 2010.96 and 2009.75) cannot be reproduced. It is thus not surprising that also our measurement is at odds with the PA and separation predicted for epoch 2015.07 using the elements by Mason (1997). Clearly, a reevaluation of all available measurements for 31 Ari is needed.

IRC+20090 was found to have ϕ_UD = 3.7 ± 0.3 mas by Tej & Chandrasekhar (2000) from a LO event in the near-IR. The S/N in our light curve is insufficient to measure the angular diameter.

The two stars SAO 96977 and SAO 96978 are the components of the wide binary system ADS 6146. We recorded both LO events inside the same sub-window, about 5 s apart, and we treated them separately. Both were found to be unresolved.

The bright star SAO 98400 (HR 3635) is an intriguing case. It was claimed as a possible double, seen in two colors simultaneously, by Eitter & Beavers (1979). They mentioned strong noise in their data and did not provide details about the separation, but they quoted a private communication from another observer who had also detected SAO 98400 as binary. Using speckle at a 4 m telescope, McAlister et al. (1989) detected the companion with 89 mas separation along PA = 71°. Following that however, the source was not detected again by speckle in several attempts including with the russian 6 m telescope (McAlister et al. 1993; Balega et al. 1994; Fu et al. 1997). The star is at just under 40 pc distance and one could speculate a highly eccentric orbit which over the course of a few years brought the component below the diffraction limits of the telescopes employed. We note that our LO light curve had sufficient S/N and sampling rate to detect the companion if the projected separation had been ≳5 mas.

SAO 96634 was reported as binary from visual micrometer observations by Couteau (1966, 1975), with separation 71 mas and PA = 438, unchanged over nine years. The system was rediscovered by Africano et al. (1978, apparently unaware of Couteau's work) who recorded two LO events, each in two colors. The projected separations (03 and 02) however did not agree with Couteau's results, and also not between themselves. Finally, Mason (1996) could not resolve the system by speckle observations at a 3.6 m telescope. Our data are consistent with a companion with 60 ms projected separation along PA = 85°, in approximate agreement with Couteau's values. However, we do not list this among our binary detections because there is no significant χ² improvement over a point-source model. The companion should be detectable by adaptive optics imaging.

IRC-20529 was found to have an angular diameter of 1.98 ± 0.35 mas by Ridgway et al. (1979), from a LO recorded in the near-IR. Our data are not inconsistent with their result, but the quality is insufficient to discriminate between an unresolved source and one with about 2 mas angular diameter.