Método

UZ Boo w as originally classified as a U Gem d w a rf nova an d w as found to h av e a m inim um m agnitude of mpg ~19.1 and a n o u tb u rst am p litu d e of ~ 3 m ag­ nitu d es (c.f Howell & Szkody 1988). The correct position of th is s ta r is given in DS93 an d inspection of a digitized PO SS p late shows a s ta r ju s t visible a t ~ 1 9 th m agnitude, a t th is position.

Previous photom etric observations of UZ Boo w hile in quiescence a re r e ­ ported in Szkody (1987) and Howell & Szkody (1988). Szkody (1987) found th a t th e s ta r exhibits ^ 0 .6 m agnitude m odulations which p otentially indicated th a t it h a s a n orbital period of n e a r 3 hours. However, Howell & Szkody (1988) found th a t th e ir V b an d lightcurve, which also covered ~ 3 hours, did n o t show th e previously reported m odulations. This led Howell & Szkody to suggest th a t th e featu res seen by Szkody (1987) w ere random , an d n o t orbital, in n a ­ tu re.

I obtained R b an d photom etry of UZ Boo over a period of ~ 3 .5 hours, w hich is sim ilar to th e tim ebase covered by Szkody (1987) and Howell & Szkody (1988). The R b an d differential lightcurve, shown in Fig. 5.1, shows no evidence for any periodic featu re or variability. I find th a t th e rm s v a ria tio n of th e V-C d a ta is ± 0.04 m ag which is sim ilar to th e calculated value of th e (rm s) noise firom th e C—K d ata. T his im plies th a t, in th e R band, UZ Boo exhibited no v a ri­ ability, w hich is indicated by th e re s u lt of th e F—te s t (c.f. tab le 5.2).

My V b a n d differential lightcurve, which spans ~ 3 .7 hours, is p resen ted in Fig. 5.2. In co n trast to th e R b an d d a ta an d th e previous photom etry of Szkody (1987) an d Howell & Szkody (1988), th e V-C d a ta show v ariab ility w ith recu r­ rin g sinusoidal-like m odulations of ~0.1 m ag in am plitude. The rm s noise in th e V-C d a ta is predicted to be 0.02 m agnitudes and th e re s u lt of th e F -te st

C hapter 5 165 ■i 030 0.750 0.72Î 0.643 0.617 0.0 1:

Î

Frequency (per hour)

Figure 5.1: UZ Boo R b an d differential lightcurve an d Lomb-Scargle p e ri­ odogram. No periodic featu res are evident.

Chapter 5 166 0.15 0.63 0.69 0.72 HID 2449759+ 0.75 0.63 0.66 12 P = 1.51 hours 10 I 0.01

8.9C

i :

Frequency (per hour)

4

1

Figure 5.2: UZ Boo V b an d differential lightcurve an d Lomb-Scargle p e ri­ odogram. M odulations a re clearly seen an d a re found to recu r w ith a period of 1.51 hours.

Chapter 5 167 0.15 0.20 ê 0.25 çj > 0.30 0.35 1.4 1.2 1.0 0.6 0.8 0.2 0.4 0.0 Phase

Figure 5.3: UZ Boo V b and differential lightcurve folded on th e 1.51 h o u r pho­ tom etric period.

C h a p te r s 168

shows th a t th e s ta r is v ariable a t >95% level. The re su lt of th e Lomb-Scargle period an aly sis of th e V b an d V-C d a ta is shown in Fig. 5.2, w here th e m ost dom inant period is found to be P=1.51 ± 0.26 hours which is significant a t > 99% level. Figure 5.3 shows th e V ban d V-C lightcurve folded on th e 1.51 h o u r p e ­ riod an d a sinusoid of sem i-am plitude of 0.03 m agnitudes w hich gives a b e st least-sq u ares fit to th e d a ta is also shown.

A question posed by th e resu lts of th e variab ility analysis is why v ariab ility is seen in th e V b an d b u t not in th e R b an d data? Inspection of th e R b a n d d a ta reveals th a t th e sky background level w as h ig h er by a factor of ~ 8 com pared to w hen th e V b an d d a ta w ere obtained due to th e presence of th e Moon. T his consequently resu lted in a lower signal-to-noise ratio for UZ Boo, lead in g to th e variance of th e V-C d a ta being higher th a n th e expected (V) low a m p litu d e variations.

T urning now to th e V b an d differential d ata, th e lack of a n observed eclipse in th e V-C lightcurve, in addition to th e available published d a ta of th e s ta r du rin g quiescence (Howell & Szkody 1988) suggests th a t UZ Boo is n o t a hig h inclination system (i.e. i is less th a n ^65°). For low, or m oderate, inclination sh o rt period system s, th e b rig h t spot contributes to th e B,V an d R photom et­ ric b an d s over all orbital phases. The sinusoidal m odulations th u s arise a s th e b rig h t spot comes into view of th e observer, reaching its m axim um b rig h tn e ss w hen it is closest to th e observer an d decreasing in b rig h tn ess a s it ro ta te s out of view (c.f. C h ap ter 1 ). An altern ativ e explanation, however, is th a t th e m od­ ulations m ay be a ttrib u ta b le to view ing different aspects of th e (heated) in n e r irra d ia te d surface of th e (distorted) secondary star. R adial velocity m easu re­ m en ts a re therefore required to distinguish betw een th e above possibilities.

Chapter 5 169

observations of a (rare) su p ero u tb u rst in 1994, reportedly its first since 1970 w here it w as observed to reach a m agnitude of ~11 (V S N E T an d th e A stronom er netw ork). D ue to short nightly coverage of th e s ta rs’ superoutburst, th e super­ hum p period w as not accurately m easured. However, based on th e available observations, a superhum p period of 1.49 hours w as estim ated (Kato 1996; p ri­ v ate comm unication). A lthough superhum ps re p e a t w ith a period w hich a re longer th a n th e orbital period by 1-9% (e.g. W arner 1996; M olnar & Kobul­ nicky 1992), th e superhum p period m easured by K ato is in g eneral agreem ent w ith, an d w ith in th e error b a r of, th e period th a t I have m easu red from my qui­ escent V b a n d V-C lightcurve. T hus th e m odulations which a re observed in my V b an d d a ta a re m ost likely to be orbital in n atu re, arisin g from th e b rig h t spot. My m easu red orbital period of 1.51 hours, in addition to th e sum perhum ps which w ere observed d uring th e sta rs’ recent outburst, support th e classifica­ tio n of UZ Boo as a m em ber of th e SU UM a group of CV.