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BJD_TDB vs HJD?

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wvinton
wvinton's picture
BJD_TDB vs HJD?

During the exoplanet course the desirability of transforming all observation times into BJD_TDB was emphasized. I understand the rationale for gathering data in a form that will be consistently invariant with respect to the reference frame of any observer.

My observation - none of the exoplanet websites seem to use BJD_TDB, for example the Exoplanet Transit Database (ETD - http://var2.astro.cz/ETD/observers.php) seems to require HJD for data submissions. Even AAVSO seems to expect HJD. The NASA website seems to report information in JD.

So, given that HJD seems to be what most sites expect, should we be transforming data into BJD_TDB or HJD? Who actually makes use of BJD_TDB?

Thanks.

PVEA
PVEA's picture
BJD_TDB vs HJD?

The Kepler mission data are in BJD TDB as the most accurate time stamp.

"BJD in TDB (BJDTDB) is usually the best time stamp to use in practice, as it further corrects the BJDTT for all known effects on the motions, and therefore rates, of our atomic clocks". (Eastman et al., 2010):

http://adsabs.harvard.edu/abs/2010PASP..122..935E

BJD TDB should be used but historically most of the data are collected using JD and HJD respectively.

Regards,

Velimir

WBY
WBY's picture
BJD_TDB vs. HJD

As Velimir stated BJD_TDB is the most accurate time stamp. The difference vs. HJD s not merely academic. Exoplanet discovery using timing of stellar pulsations, for example, must use observations based on BJD_TDB. The complicated motion of the sun with respect to an inertial reference frame (at least as close as we can approximate) would completely frustrate this technique if not corrected out of observation timing. The accuracy of observation time stamping (in whichever known time reference) is also critical in making these observations. A plot of the Sun's "fandango" around the barycenter is attached.  Of course, you also have to account for the long term proper motion between the barycenter and the target and any long term evolutionary changes in pulsation rate. 

By long term observations of pulsating white dwarf stars, timing precision of better than 1 x 10E-14 s/s has been achieved and at one time provided the most precise validation of the NIST time standard (I don't know if another method, such as pulsar observations, has subsequently provided more precise validation). Timing precision is sufficiently good that the rate of change of pulsation in white dwarfs due to cooling has been measured at 2 x 10E-15 s/s, as shown in the left-hand plot in Fig. 1 of the attached paper.  

Brad Walter, WBY

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