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LPV of the month - June 2020- X Her

X Herculis

By Rich Roberts

     Have you ever been casually acquainted with someone for years, but you never really took the time to get to know them? Then some event brings you together and you discover they are actually really interesting. After this discovery, you decide you want a closer relationship and begin to ask yourself why you didn’t pay more attention to them before. This is how I feel about Hercules. I knew Hercules was there, but I never paid it much mind. Its naked-eye stars are dim and easily overlooked with Lyra, the Northern Crown, and Bootes all nearby. Even after getting my first telescope, I didn’t find myself that interested in Hercules. Sure, it is home to a fairly good glob and a fairly okay glob, but what else of interest is in its vast expanse? I preferred to head east into Lyra and look at the Ring than mess around in Hercules. This is how I felt. I didn’t appreciate Hercules until I became a variable observer and could then fully understand the depth and complexity of its character.

            Hercules is filled with many bright Miras. As a visual observer, I focus on well-observed LPVs with long histories, because that is where I can do the most good. LPVs all have their own idiosyncrasies, which is why I write these articles. However, Hercules is home to many other interesting types of variables as well. RCB type stars are very rare, but Hercules has one in AO Her. Z Cam stars are fun, and Hercules has one of those too with AH Her. If your thing is chaotic RVAs, go to AC Her. Like YSOs, observe V117 Her (I reiterate this is a freakin’ cool star; Michael Poxon wrote about it at Hercules is also home to the prototype magnetic polar variable, AM Her. Hercules has something for everybody, you just need to look beneath the surface and not judge a book by its cover. We’ll do so today by examining one of Hercules’s many interesting LPVs, X Her.

     X Her is an oxygen-rich (M6e) AGB star classified as a SRb in VSX and the GCVS. X Her varies between 5.8 and 7.0, making it a good target for binocular observers and those with small telescopes. It is well observed, with 56,478 total observations in the AID (97.8% of which are visual) dating back to April 2nd, 1891; however, coverage was sporadic until 1946. Frank Vohla (VFX) has registered the most observations with 2,327. Steven Sharpe (SHS) is second with 1,611 observations (Vis, TG, & V), followed by Edward Oravec (OV), who logged 1,484 visual observations. Seven observers have observed the star more than 1,000 times.

            X Her’s light curve is fairly sporadic. This is to be expected with this type of star, as SRb stars are defined to have poorly expressed periodicity (Percy 2007). The light curve below is 650 days-worth of visual and Johnson V observations. The trend line is a 7-day binned means on the visual observations.

And a 1000-day light curve from ASAS-SN:

To say X Her’s periodicity is poorly expressed would be an understatement. Multiperodicity is not uncommon for this type of star; however, there tends to be some agreement at least on what the two periods are. The GCVS lists two periods for X Her, a primary of 95 days and a secondary of 746 days. However, VSX list the two periods as 102 and 178 days, respectively, which agree with the findings of Kiss et al. (99). Different literature sources list different periods, including Mathews et al. (2011), which reports three different periods—so who knows that this star is really doing. I ran a Fourier analysis on the visual data between 1950 and today with VStar and got the following unenlightening results. The top hits were 101.4 days and 176.5 days, although many strong frequencies appear.

All of the top hits created some of the worst-looking phase plots (for both visual and photometric observations) I have ever seen. When looking at the period over time, you see strength switching between the 102-day period and the 178-day period, but the results are simply not that conclusive.

Semi-amplitude seems to be varying over time as well. So, all in all, this star is a little on the eccentric side and just does its own thing. A true free spirt!

     X Her may have recently evolved on to the AGB since its carbon to oxygen ratio is less than one and no technetium (Tc) has been found in its spectrum, which indicates thermal pulse dredge-ups have yet to occur. X Her is 1.9 times more massive than the sun and 3,510 times more luminous. If X Her was placed in our solar system, its radius would extend 0.1 AUs past the orbit of Venus. However, X Her’s extended structures don’t stop there. X Her has a circumstellar envelope (CSE) surrounding the star, which contains about 700 Earth masses worth of matter. This CSE may actually consist of two separate shells due to varying strengths of its stellar wind. Matthews et al. (2011) studied the H1 regions around the star and found a head- and tail-like structure (~0.24 pc long), similar to those found around Mira and RS Cnc. There is also a nearby hydrogen cloud with approximately 2.4 solar masses of material that could be interacting with X Her’s unusual shell.

Jorrisson et al. (2011) also studied X Her’s interactions with the ISM, utilizing the PACS instrument onboard the Hershel Satellite, and found a heart-shaped structure (shown below).

X Her’s unusual axial-symmetry is likely due to several sources. For one, X Her is a “high velocity star,” traveling at about 73 km/s against the local standard of rest. Jimenez-Estaban et al. (2012) looked for very red stars with high proper motions in Tycho-2 and 2Mass data. X Her had the 4th highest proper motion (94 +/- 1.1 mas/a) behind Mira, L2 Pup, and V744 Cen.  X Her also experiences bi-polar molecular outflows and H2O Masers. Research papers cite mass-flow rates which can vary by more than an order of magnitude, so really X Her has a lot going on.

     Observers should continue to monitor X Her. It has reasonable comparison stars for visual observers to provide good observations. With everything going on with this star, continuing to build a well-defined light curve to monitor the period changes will be interesting. Plus, it’s in a vibrant, exciting neighborhood. Now that Hercules is starting to rise into a good observing position after sunset, I hope everyone reading this will give the constellation the love and attention it deserves, and won’t forget to drop in on X Her while there.
























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