Mon, 01/09/2017 - 17:59
Hello everyone,
My name is Javier and I´m an IB student from Spain. I´m doing my monograph about the relationship between globular clusters´ peculiar velocity and galactic latitude. In order to do this, I need to calculate the distance to some globular clusters close to the galactic bulbe, but I don´t know how to calculate distances with RRLyrae data of passband I, Ic, pg and B. Could you teach me how?
I have no much time to finish my work, so I would be grateful if you answer me as soon as possible.
PD: I´m using data from catalog B/vsx.
Thanks for your atention,
Javier
If this is the catalog Jaiver is refering too, are there RR Lyrae stars in these open clusters? And how would you know they belong with the cluster?
http://cdsarc.u-strasbg.fr/viz-bin/Cat?J/A+A/548/A97
http://adsabs.harvard.edu/abs/2012yCat..35480097Z
Rodney
There aren´t many clusters where I could find RRLyrae, but I found them in some. However, when they are close to the bulbe (low galactic latitudes) the catalog don´t use data of V or CV passband, so I need the method to calculate them.
I haven´t much experience in astronomy research, so I seek for advice here. Really I am not sure if the RRLyrae I´ve used belong to the cluster, but I deduce it because of their coordinates and the very similar V magnitude values they have.
PD: I have dismissed some stars because of their anomalous data. But I can´t be too demanding with data because I need an important number to draw conclusions.
Javier
Perhaps someone with more experience with RR Lyrae stars could help with this?
Perhaps Javier, you could post the 'catalog' of stars you have identified as being in these clusters?
Rodney
Also, Is this thread in the right forum??
AAVSO recommended me this forum, but I´m not sure which is the most appropiate. Could you give me some advice?
As you probably know it can be difficult to work with bulge clusters because of crowding of the images with many non-members and because interstellar reddening can be high and sometimes variable across the cluster. In addition, the more metal rich bulge clusters often have red horizontal branches that lie mainly to the cool side of the instability strip, and therefore have few RR Lyrae stars. Perhaps the best way to examine what people have done with bulge clusters is to look at some of the recent papers where authors have derived distances to individual clusters in the bulge. A number of such papers are available through the NASA Astrophysics Data System, for example those on NGC 6401 and M62. The distance determinations are usually tied to an adopted relationship between RR Lyrae metal abundance and absolute magnitude. Generally, the more metal-poor RR Lyrae stars are more luminous than the metal-rich ones, but there are exceptions, as in the oddball clusters NGC 6388 and NGC 6441. As results from the GAIA mission come in, we should have a better handle on a number of these problems.
Thank you very much for the information. Then, there is no method or equation to calculate distances with máx and min of passband Ic, I, B or pg, although they aren´t close to the galactic bulge, or I can´t calculate this because the huge uncertainty and reddening of galactic bulge . What really interest me is the method, equation or the established value for the absolute magnitude of these passbands that would allow me to calculate distances to clusters with data of these passbands. The distance to the bulge is also important, but, after your explanations, I understand perfectly why I shouldn´t choose clusters close to the bulge if I am not sure of the other important facts.
Javier
Sorry, maybe I haven´t expressed myself well.
There are various relations that have been published between RR Lyrae absolute magnitudes in different passbands and, usually, metal abundance and/or period (though, as noted before, these relations may not be universally applicable). For example, one set of theoretical relations can be found at: http://adsabs.harvard.edu/abs/2004ApJS..154..633C . The old photographic, pg, magnitudes are not used in modern observations, so you might have difficulty finding a modern calibration for blue observations other than B. Also, for mean magnitudes, astronomers now usually use a fit to the whole light curve and not just an average of the maximum and minimum magnitudes. While getting the very best distances to bulge globular clusters can be messy, you may nonetheless want to try to see how well your results agree with those in the literature.