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When Hubble first discovered a Cepheid variable in the galaxy M31, the universe grew. Previously, many astronomers had held that the fuzzy “spiral nebulae” were small patches of gas and dust within our own galaxy, but through the Period-Luminosity relationship which allowed him to determine the distance, Hubble demonstrated that these were “island universes”, or galaxies in their own right.
Soon after, Hubble (as well as other astronomers) began searching other fuzzy patches for Cepheids. Among them was the spiral galaxy M33 in which he discovered 35 Cepheids. Among them was V19 which had a 54.7 day period, an average magnitude of 19.59 ± 0.23 MB, and an amplitude of 1.1 magnitudes. But according to recent work revealed at the recent American Astronomical Society meeting, V19 no longer seems to be pulsating as a Cepheid.
The new research uses observations from the 3.5m Wisconsin, Indiana, Yale, and NOAO (WIYN) Observatory as well as the 1.3m Robotically Controlled Telescope (RCT) operated jointly by a group of universities and research institutions. The new observations confirm a 2001 report that found V19 had decreased its brightness amplitude to at least less than 10% of the magnitude reported by Hubble in 1926, and possibly further as any fluctuations were below the threshold detectable by the instruments.
Now, if any variation exists, it is less than 0.1 magnitudes. The new study reports that there may be some small fluctuations, but due to inherent uncertainty in the observations, it barely exceeds the background noise and the announcers did not commit to these findings. Instead, they pledged to continue observations with larger instruments to the equation to push down the instrumental error as well as adding spectroscopic measurements to investigate other changes in the star. Another of the peculiar changes V19 has undergone is an increase of about half of a magnitude to 19.08 ± 0.05.
These changes are strikingly similar to another, more famous star: Polaris. Due to its much closer nature, observations have been much more frequent and with lower detection thresholds. This star had previously been reported to have an amplitude of 0.1 magnitudes which, according to a 2004 study, had decreased to 0.03 magnitudes. Additionally, based on ancient records, astronomers have estimated that Polaris has also brightened about a full magnitude in the past 2,000 years.
According to Edward Guinan of Villanova University and one of the members of the new observational team, “both stars are experiencing unexpectedly fast and large changes in their pulsation properties and brightness that are not yet explained by theory.”
The primary explanation for this dramatic change is simple evolution: As the stars have aged, they have moved out of the instability strip, a region on the HR diagram in which stars are prone to pulsations. But these stars may not be entirely lost from the family of periodic variables. In 2008, a study led by Hans Bruntt of the University of Sidney suggested that Polaris’ amplitude may be increasing. The team found that from 2003 to 2006, the scale of the oscillations had increased by 30%.
This has led other astronomers to suspect that there may be an additional effect in play in Cepheids known as the Blazhko Effect. This effect, often seen in RR Lyrae stars (another type of periodic variables), is a periodic variation of the variation. While no firm explanation exists for this effect, astronomers have suggested that it may be due to multiple pulsational modes that interfere constructively and destructively and occasionally forming resonances.
Ultimately, these strange changes in brightness are unexplained and will require astronomers to have to carefully monitor these stars, as well as other Cepheids to search for causes.
I presume that if a Cepheid variable changes it frequency it also changes it luminosity more or less within the P = f(L), for P and L periodicity and luminosity.
LC
This article is right in saying stellar evolution is likely the cause. The narrow instability strip for large c.8 solar mass stars can a few thousand years at best.
This is why in an H-R Diagram we see all the stars on left hand line of the main sequence and the red giants on the upper right hand side. In between finds the so-called Hertzsprung gap, where few stars reside. It explains that for 90% of the time sits on the main sequence, but when it starts running out of hydrogen fuel, the star rapidly swells into a red giant. It is thought in this gap the star physical structure of the star evolves — no only does the star swell in size, the core shrinks and the way convection in the bulk of the star changes too. Pulsations occur as the energy source oscillates, probably as the core become active when the core is crushed by gravity, then the energy released by the active phase, expands the star again, where it becomes less active. Continuous oscillations between the two states are magnified and echo throughout the outer regions of the star, which we see as pulsations. (The best analogue is breathing in and out.)
I read somewhere (Iben Jr., I think), the general explanation for switching off star like Cepheids in the instability strip is due to two different or separate harmonic oscillating phases occurring at once, and these can cause interference and cancel out each. For a while the pulsations stop, and least until one of the pulsations period again dominates. The the regular pulsations then begin again. The star might do this several time while in the region of the instability strip.
Evidence for these complex harmonic oscillations can be seen in the ZZ Ceti stars, white dwarf crossing their own instability strip. Here dozens of periods can occur at once, which when the “waves” combine or peak together, can cause sudden violent starquakes.
As for the star brightening, the cause is likely the dramatic increase of luminosity as it moves to a red giant. I’d assume the pause while crossing the instability strip somehow inhibits the total luminosity, but when the pulsations holt, the luminosity suddenly jumps. Here, once the radiative transfer goes from a chaotic state to a organised one, whose efficiency causes the star to become brighter.
In the end, I think that understanding the complex harmonics occurring in stellar atmosphere might leave an adequate explanation of the phenomena.
It is my weak understanding of stellar astrophysics that these oscillations are due to some convection or outgassing of helium. It them makes some sense that Cepheid variables are older stars near their end.
LC
Thanks, interesting and helpful!
The second story in two days about how our studies of Cepheids must be made more precise. The first one can be found here.
With this and INTEGRAL noticing dimming of the Crab Nebula remnant our standard candles need some careful checking, Hopefully they will still be useful enough and it does seem to provide more info on what happens in the instability strip and a better insight into stellar evolution.
This article is a tad confusing for outsiders. I assume for whatever historical reason the amplitude of an average (bias, “dc”) signal isn’t an astronomical “amplitude” but the amplitude of the modulation (“ac”) is.
This could explain why this sentence hangs out there after “V19 had decreased its brightness amplitude to at least less than 10% of the [previous modulation?] magnitude”: “Another of the peculiar changes V19 has undergone is an increase of about half of a magnitude to 19.08 ± 0.05.”
It doesn’t explain why the decrease is an “increase”, though. 😀