AUTHOR=Evans Nancy Remage , Engle Scott 

TITLE=Prospects for X-Ray and FUV Observations to Provide Insight Into the “Cepheid Mass Problem”

JOURNAL=Frontiers in Astronomy and Space Sciences

VOLUME=Volume 6 - 2019

YEAR=2019

URL=https://www.frontiersin.org/journals/astronomy-and-space-sciences/articles/10.3389/fspas.2019.00065

DOI=10.3389/fspas.2019.00065

ISSN=2296-987X

ABSTRACT=The classical Cepheid Leavitt Law (Period-Luminosity relation) has been known for 
100 years, and used as in extragalactic distance determination for nearly as 
long.  However, Cepheids still present us with surprises.  Recent results about X-ray 
behavior is just one example.  Around minimum radius the pulsation wave from the 
envelope creates a disruption in the photosphere and chromosphere.  X-ray activity, 
on the other hand remains constant at a low level at this phase.  Just after maximum radius, however,
X-ray flux increases sharply (four fold in the case of $\delta$ Cep).  We summarize here 
the observational program to explore parameters involved and theoretical investigations
searching for a basic understanding of this upper atmosphere phenomena.