[vsnet-chat 4342] re V1436 Aql

[crawl@zoom.co.uk]

I doubt that this nearby red dwarf is an eclipsing binary.

Let's look at the Hipparcos data.

At V around 9.5 this object is below the nominal cut off point of that
catalogue, and is probably only included because of it being a nearby star.
 Many data entries for this star in the Hipparcos catalogue are blank,
which means the data couldn't be safely deduced, which is usually
consequent upon the faintness and/or extreme colour of a star.

This happens to be a cpm binary with a comes at around 4.2 arcsec [1985].
This means that both stars will have been in the 10 arcsec field of the
Hipparcos instrumentation within in which objects cannot be seen
separately.  Given the already noted fact that this object is below the
nominal limiting magnitude of the Hipparcos experiment, and that despite
this the same experiment is capable of picking up photons from the 12th mag
comes, which being so faint would not have had a separate solution, and
also that the bright star itself is quite red at B-V about 1.8, again a bit
of a problem area for faint end Hipparcos objects, you have to start
worrying about the assumption of eclipses.  Chris Lloyd has already noted
difficulties with the epoch photometry.

ANYWAY, let's look at the Tycho data, from the independent star mapping
instrumentation of the Tycho experiment.

The Tycho experiment had a limiting magnitude below the V about 9.5 for
this object so there are is no problem in that regard, and indeed the
Tycho1 catalogue lists it as having a scatter in VT of only 0.28 mags,
which could more or less be noise as far as the Tycho experiment is
concerned, and nothing compared to the 2 mags range listed in the Hipparcos
Catalogue.  Tycho2 actually gives the standard error on the VT magnitudes
as being _0.029_, which is even less!  Interestingly, BT is about 11.4, or
near what Hipparcos quotes as being the minimum in Hp... ...a typo crept in
somewhere?

Unfortunately the Tycho1 epoch photometry for this star is in annex B,
which is only accessible via the CDS, so I'll have to order that.  When I
get it I'll forward it to vs-chat.  Tycho epoch photometry is two colour,
BT and VT, and most EA type eclipsing binaries show colour changes during
eclipse, so that might be interesting.  As I say though, I can't think of
any red dwarf eclipsing binaries, at least no large amplitude ones.  I'm
willing to stand corrected.

Doesn't stop the star being variable though, although flaring is the only
activity I'm aware of for red dwarfs.

IMPORTANT : when folk use Hipparcos data they should really ensure that
they take into account _all_ the data for that object in the main
catalogue, including all the flags, which have significant bearing on the
quality and nature of the data, and stop taking isolated chunks of data and
treating it as sacrosanct.  It is often also a thought to check what the
Tycho data for the object says as well, not only because that experiment
used independent instrumentation and data processing, but also because it
gives two colour photometry, frequently with two colour epoch photmetry for
interesting objects, and thus enables a deeper assessment of the objects
nature.

Cheers

John

John Greaves, UK

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