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June 6, 1999

    Digging out unknown variable stars    

MISAO Project Announce Mail (June 6, 1999)

Hello. I am Seiichi Yoshida working on the MISAO project.

Discovery of the first new variable star of the MISAO Project, MisV0001, was announced on Apr. 3. Then new variable stars survey continues while taking new CCD images. Until June 1, 32 new variable stars have been discovered. The image showing the variable behavior and all magnitude data are available for each at the MISAO Project Home Page:


I enclose the table of all new variable stars at the end of this mail. We can see some interesting facts from this table.

First of all, there is an evident bias among the new variable stars. Actually, 24 stars of the 32 were discovered from the image of Nova Sco 1998. And 4 stars of the rest 8 were from the image of V4334 Sgr (Sakurai's object).

In order to discover variable stars, two images of the same area, of the different date, are required. However, the Ageo Survey Team (KenIchi Kadota and Seiichi Yoshida) has just begun the regular survey in early April. So we have only few images satisfying the condition. The image of Nova Sco 1998 and V4334 Sgr are the rare exceptions. Therefore, almost all the new variable stars discovered so far are limited to those areas.

The size of those images is only 0.8x0.5 deg, or 1.5x1.0 deg. But many new variable stars are discovered from such small areas unexpectedly. This implies many variable stars remain unknown all over the sky.

The new variable stars discovered in the course of the MISAO Project are 11-14 mag at peak. Many of them should be Mira-type. Mira-type variable stars are red in general, so it looks brighter by CCD. In the experience of the observations of known Mira-type variable stars, the magnitude of them by CCD becomes 2 or 3 mag brighter than visual observation. It means, these new variable stars are very faint, whose peak magnitude are about 14-17 mag visually or by photo. This will be one of the reasons why many Mira-type variable stars are remained unknown after the photo survey.

There is a big gap between the number of new variable stars discovered from Nova Sco 1998 image and V4334 Sgr image. The number of stars detected by the PIXY system are about 6000 and about 3500 for each. So the gap is almost twice, but the gap of the number of new variable stars is bigger, about 6:1. The reason should be the difference of observation period, I guess. The period of Nova Sco 1998 observation is from Jan. 30 to May 7, about 3.5 months. On the other hand, the period of V4334 Sgr observation is from Feb. 12 to Apr. 29, about 2.5 months. Actually, the instruments on Feb. 12 and Apr. 29 are differ. Limiting to the same instruments, the period is until Apr. 13, only about 2 months.

In the experience of the MISAO Project, only few stars behave variable between images of one month period. A few stars behave variable between images of two month period. But many stars behave variable between images of four month period. It will be because the period of a Mira-type variable star is about 300 days.

As a conclusion, when you can take CCD images in the Milky Way and catch stars as faint as 14-16 mag, you can discover many new variable stars. All the new variable stars of the MISAO Project were discovered with 0.18-m or smaller amateur telescopes, with about 20-sec or 1-min exposure. If you have the same kind of telescope, please take CCD images of your favorite area in the Milky Way, take images again of the same area after several months, and send the images to the MISAO Project. I guess we can find some new variable stars immediately. The new variable stars will be registered as MisV objects and announced to the whole world referring to your name as an image observer.

Especially, asteroids survey images will have enormous unknown variable stars, I guess. Actually, after investigating the images of the MISAO Project, few asteroids were detected, not only new ones but known ones. It implies that the number of new variable stars are much larger than that of asteroids. Probably, in the case of asteroids survey images, they only compare two images of the same area with a short time gap and check if only moving objects exist or not. But many new asteroids are discovered from their survey images actually. So much more new variable stars must be detected when the images are checked for new variable stars.

The Ageo Survey Team, KenIchi Kadota and Seiichi Yoshida, has just begun the regular survey with 0.16-m f/3.3 reflector since early April. Some new variable stars have been discovered so far from those images. But because only two months have passed, most of the areas are taken only at once. Even if an area has been taken twice or more, the period is too short for Mira-type variable stars to behave variable evidently. Therefore, after next summer, when the observation period reaches to 4 months, many new variable stars will be discovered from those images.

By the way, the Ageo Survey Team has been enthusiastic in taking comets or supernovae. We kept observing the same area for several months when the supernova is bright. But no new variable stars has been found from supernova images. That is because the stars are not crowded at all around a supernova in general. Around the Nova Sco 1998, about 6000 stars are detected by the PIXY system. But in the case of supernova images, only 200-400 stars are detected. So the survey efficiency is only 3-7 percent.

The past MISAO project announce mails are available at:


* Table of new variable stars discovered by the MISAO Project

Code     R.A. (J2000.0) Decl.  Max    Min   ID               
MisV0001 175226.59 -174000.7  13.9C  16.9C  
MisV0002 072403.56 +412602.0  13.3C  14.7C  GSC2965.0210 USNO1275.06972791
MisV0003 175244.93 -172401.4  12.5C  13.4C  
MisV0004 175313.59 -172844.2  12.1C  13.9C  
MisV0005 165928.08 -132314.1  12.3C  14.1C  USNO0750.10268837
MisV0006 175605.00 -311520.9  11.7C  12.9C  
MisV0007 170005.99 -160414.3  12.7C  13.8C  USNO0675.11655030
MisV0008 175553.96 -311443.2  14.1C  14.9C  
MisV0009 185911.39 -013410.9  11.5C  13.4C  USNO0825.13934695 IRAS18565-0138
MisV0010 175513.12 -311451.8  13.2C  15.8C  USNO0525.24816214
MisV0011 175040.92 -174038.1  12.0C  13.6C  IRAS17477-1739
MisV0012 175644.26 -310401.5  11.9C  13.1C  IRAS17534-3103
MisV0013 175648.87 -310148.7  12.4C  14.5C  
MisV0014 175637.90 -310045.7  12.6C  16.3C  
MisV0015 175626.80 -311101.1  12.9C  14.3C  USNO0525.24950305
MisV0016 175639.69 -305928.7  13.0C  14.8C  
MisV0017 175722.16 -305457.3  11.2C  14.6C  
MisV0018 175448.31 -310220.2  11.8C  15.0C  
MisV0019 175456.77 -310225.8  14.0C <16.5C  IRAS17516-3101
MisV0020 175452.49 -310248.3  13.4C  15.3C  
MisV0021 175531.57 -310523.0  11.9C  14.5C  
MisV0022 175528.00 -310425.0  12.6C  17.1C  
MisV0023 175553.13 -310224.8  15.1C  16.4C  
MisV0024 175558.13 -304710.7  13.3C  16.5C  
MisV0025 175633.46 -304629.0  13.3C  14.6C  
MisV0026 175708.98 -305822.8  13.6C  14.6C  
MisV0027 175444.64 -310539.9  13.0C <16.5C  
MisV0028 175517.97 -310032.8  13.8C <16.0C  
MisV0029 175444.64 -305340.2  13.2C <15.5C  
MisV0030 175637.14 -305105.4  14.7C  16.4C  USNO0525.24967963
MisV0031 175424.43 -310534.1  14.5C  16.2C  USNO0525.24729694
MisV0032 175437.67 -305327.4  13.9C  16.9C  
Seiichi Yoshida

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