Calculating Bolometric Correction for Cepheid Variables

[toph]

hi

can the bolometric correction be calculated if all i know of a cepheid variables properties are its period? for example, if the cepheid variable has a period of 3 days i can use a period-luminosity relationship to calculate the luminosity, from here i can then calculate the bolometric magnitude. my question really is this, can i then calculate the bolometric correction required to find the absolute visual magnitude? or is there an easier way to get to the absolute visual magnitude from just the information given.

hopefully some one can point me in the right direction, thanks

(this is a question that has come up in one of the books i am reading, so if it is in the wrong forum i appologies)

 

[fasterthanjoao]

As far as I know, you need to have photon counts for specific temperatures for the stars that you want to calculate the correction for. This is basically because at a given temperature, there is a photon count we expect to see and a photon count we actually see, the difference is, in part, due to the fact that the detector we use is not sensitive to all wavelengths.


The formula I'm aware of regarding bolometric correction is:

<br /> <br /> BC = 2.5*\log(\frac{N_1}{N_2}) - 10\log(\frac{T_1}{T_2}) - BC_2<br /> <br />

where Ni's are photon counts and T's are temperatures. BC2 is the bolometric correction for the second star.

Though, I'm not sure if you need to, but there are other factors worth metioning. For example, the detector used to find the counts above is likely to not only not be sensitive to all wavelengths but is also likely to unintentionally incorporate a bias - it will not be equally sensitive to all wavelengths. There are also other factors such as - are your photon counts subject to interstellar absorption? Not sure if any of this is relevant.

 

[Labguy]

hi

can the bolometric correction be calculated if all i know of a cepheid variables properties are its period? for example, if the cepheid variable has a period of 3 days i can use a period-luminosity relationship to calculate the luminosity, from here i can then calculate the bolometric magnitude. my question really is this, can i then calculate the bolometric correction required to find the absolute visual magnitude? or is there an easier way to get to the absolute visual magnitude from just the information given.

hopefully some one can point me in the right direction, thanks

(this is a question that has come up in one of the books i am reading, so if it is in the wrong forum i appologies)

For a Cepheid, if you know nothing but the period, you wouldn't be able to use the period-luminosity relationship to get bolometric magnitude or much of anything, including distance. This is because there are "classical cepheids" of Population I with periods of 2 to 40 days and "W-Virginis Stars" which are cepheids of Population II.

For a given period, the W-Virginis stars are about 1.5 to 2.0 magnitudes less than classical cepheids, so you would at least need a spectral analysis to determine if the star is a Pop I or Pop II cepheid. It was on this particular point that the early (1920's I think) measurements to the Andromeda galaxy showed ~1.1 million Ly when it is actually ~2.2-2.5 million Ly. The first cepheid measurements and period-luminosity relationships were done only on classical cepheids, hence the original error(s).

 

[toph]

thank you

that kind of makes sense, i have seen several tables showing the bolometric correction, these vary with spectral type so i can see why the temperature would be important. Do you then know of any way to estimate the absolute visual magnitude from just the peiod of a cepheid variables? I am particularly interested in two stars with periods of 3 and 60 days respectively.

 

[Labguy]

thank you

that kind of makes sense, i have seen several tables showing the bolometric correction, these vary with spectral type so i can see why the temperature would be important. Do you then know of any way to estimate the absolute visual magnitude from just the peiod of a cepheid variables? I am particularly interested in two stars with periods of 3 and 60 days respectively.

No I don't, without knowing the spectral type. The W-Virginis (Pop II) cepheids cluster around 18 day periods but some are as short as ~2 days. See:

http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1991AJ...101.1795M&amp;data_type=PDF_HIGH&amp;type=PRINTER&amp;filetype=.pdf

But, it might be safe to assume a 60 day period would be a classical (Pop I) cepheid due to that long period. I'm not sure, but I don't know if any Pop II's with long periods as 60 days.