reference stars

[Keith Graham]

Hi,

It has become apparent to me that I need some clarification on reference and catalog stars when deriving the IR curve for an observation. I try to select a reference star that is as physically close as possible to the target. I then check the stellar class of that reference star and try to get an exact match of that class with a Miles star. For example, if the reference star I select is an A5V star, I want to select an A5V star from the Miles catalog. But what if there is not an A5V star listed in Miles? Can I use, say, an A0V (or some other AxV class star) from the catalog? Just how much leeway in the number (if any) is there when using a catalog star to match the reference star? Can I assume that I should stick with the "V" in this case instead of using, say, and A5IV star?

Also, I notice that some classes have a lower case letter after the class - i.e A0Vn. What does the "n" mean. Could an A0Vn catalog star be used with an A0V reference star to derive the IR.?

Cheers,

Keith Graham

[Robin Leadbeater]

Hi Keith,

An interesting question. If the reference star you have measured is not an actual Miles star I would say it is better to use the generic Pickles spectrum of the correct type rather than a Miles spectrum of a similar star. It is worthwhile checking though that the star you have chosen as a reference does not have a high interstellar redenning. Francois Teyssier has a spreadsheet listing good low IS extinction A stars for use beyond the Miles stars which is posted here somewhere.

Cheers
Robin

[Keith Graham]

Hi Robin,

Thanks for the reply. I figured it is best to use an exact match, and this always has been my practice. But I was curious if a close match might be useable and now I know for certain it is not.

Yes, I am very familiar with Francois' spreadsheet, and it is my source for reference star determination. It is such a useful tool, and easy to use.

Cheers,

Keith

[Robin Leadbeater]

Hi Keith,

it is best to use main sequence (v) stars as they are less likely to be variable but I have on the odd occasion resorted to interpolating between Pickles spectra to produce intermediates for an exact comparison eg making an A1v by averaging the A0v and A2v spectra. In practise though if you look at most stars (eg using Brian Skiffs database http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=B/mk ), their exact classification is generally rather "fuzzy" in any case (look at Altair for example which you would think is a straightforward standard star but has been variously classified over the years as anything from A5iv to A9v), hence the advantage of using a star with an actual spectrum where possible.

Cheers
Robin

[Keith Graham]

Very good point, Robin. I always try to find a reference star in Miles that is close to the target. But, of course, we cannot always be so lucky.

Cheers,

Keith