[vsnet-chat 719] Problem in SS Cyg chart

[Taichi Kato <tkato>]

   I have noticed an interesting example in the SS Cyg field.  The star

GSC3196.1427 214444.13 +432545.0 (2000.0)  7.01 1
214446.5 +432630 (2000.0) BD+42.4195 8.5
214444.0 +432544 (2000.0) SAO51245 8.7 7.8 K5
214444.1 +432545 (2000.0) 752HV1 1.72 HIP107361 plx=1.60(0.76) (0.01 0.01) U 7.59-7.65

   is labelled as (80) most of the charts I have.  The chart in Burnham's
Celestial Handbook also marks this star as B = (80).  This value differs
as much as 0.5 mag from the Hipparcos V value!  The Hipparcos catalogue
also tells this star is likely a microvariable, although no GCVS designation
has been given yet.  From the current standpoint, this star is quite
inadequate both in terms of its large B-V, and its suspected variability
(the long-term variability may be even larger).  If this V magnitude really
represents the ensemble average of each observer's response to this star
(this assumption may be reasonable especially in the case of SS Cyg, which
has been monitored regardless of the moonlight, twilight, and poor sky
conditions), outburst brightness estimated using this comparison star may
have underestimated up to 0.5 mag!  People may argue the whole outburst
pattern may not be significantly modified in the presense of this noise,
but I must stress some quantities important to dwarf nova physics, e.g.
decline rate, may have suffered a significant systematic effect.  Even more
worse is the effect on long-term analysis, as in testing cyclic long-term
activity based on changes in outburst magnitudes or other quantities.
I presume the red color of this comparison may have resulted significant
lunar phase-dependent or seasonal biases, which may have worked as "noise"
to other possible (latent) long-term trends.  The situation in U Gem may be
even more severe.

   Interestingly, looking back to the VSOLJ record, this star was already
suspected of variability.  Some observers actually reported visual estimates
of this comparison star.  This may be taken as another evidence that this
star looked different, at least for some observers, from the "canonical"
chart magnitude (80).

Regards,
Taichi Kato

VSNET Home Page

Return to Daisaku Nogami

---

vsnet-adm@kusastro.kyoto-u.ac.jp