white dwarf – Cosmic Reflections

ZZ Ceti Stars

About 80% of all known white dwarf stars have hydrogen atmospheres, showing only hydrogen absorption lines in their spectra. These have been assigned the white dwarf spectral type of DA (presumably D for dwarf and A for the first, or most common, type of white dwarf). Arlo Landolt (1935-) was the first to discover variability in a white dwarf by observing the m_v=15.0 DA white dwarf star named HL Tau 76 (not to be confused with HL Tau!), in front of a dark nebula in Taurus (LDN 1521C = MLB 3-13), in December 1964. This star now has the standard variable star designation V411 Tauri.

A second DA white dwarf, m_v=14.2 Ross 548 in Cetus, was discovered to be variable by Barry Lasker (1939-1999) and Jim Hesser in 1970, and in 1972 it was assigned the variable star designation ZZ Ceti.

By 1976, seven luminosity-variable DA white dwarfs had been discovered, and John T. McGraw and Edward L. Robinson stated in an ApJ paper

We suggest that the recently proposed ZZ Ceti class of variable stars be reserved for the DA variables in Table 1 and specifically exclude the DB variables since the mechanism of variation is almost certainly different.

McGraw & Robinson, Astrophysical Journal, Vol. 205, p. L155-L158 (1976)

So, why wasn’t this new class of luminosity-variable DA white dwarfs named after V411 Tau, the first of its class discovered? Why are they called ZZ Ceti stars, after the second such star discovered, instead? In each constellation, variable stars are given one- or two-letter designations in order of discovery, and when the letter designations run out, the letter “V” is used followed by a number. The first “V” star is V335, the 335th variable star to be discovered in a constellation. Well, V411 Tau was the 411th variable star discovered in Taurus, and as a matter of tradition, no class of variable stars is ever named after a “V” designation. So, the honor fell to the runner-up, ZZ Ceti. Besides, ZZ Cet is a little brighter than V411 Tau, so not a bad choice.

ZZ Ceti stars, also known as DAV stars (as in DA Variable), are multimodal pulsating white dwarfs having periods ranging from 70 seconds to 25 minutes. But the amplitude of the brightness variations is tiny to small, ranging from less than 0.001 magnitude up to 0.3 magnitudes.

V411 Tau has a dozen detected pulsation modes. In order of amplitude (in millimagnitudes), they are (without error bars):

Period (seconds)	Amplitude (mmag)
540.95	30.89
382.47	17.88
664.21	16.22
596.79	15.63
1064.97	12.27
781.0	9.88
492.12	7.73
449.8	7.27
976.38	7.01
799.10	5.63
1390.84	4.26
933.64	2.61

ZZ Cet has eleven detected pulsation modes. In order of amplitude, they are (without error bars):

Period (seconds)	Amplitude (mmag)
213.1326	7.119
274.2508	4.658
212.7684	4.448
274.7745	3.368
212.9463	1.477
274.5209	1.253
186.8740	0.9518
318.0763	0.7836
333.6447	0.6740
217.8336	0.3506
318.7657	0.3289

ZZ Ceti stars are not radial pulsators, that is they do not undergo radial oscillations (changes in size). White dwarf stars typically have diameters of only 0.9 to 2.2 that of the Earth, so they are much smaller than “normal” stars. As short as the pulsation periods are, they are not short enough if the cause were radial pulsations. Instead, the pulsations are due to shock waves traversing the atmosphere of the ultradense star. The slow rotation of these stars often causes closely spaced “double periods” such as we see in ZZ Cet.

The brightest (and closest) ZZ Ceti star yet discovered is DN Draconis, shining at visual magnitude 12.2. DN Dra pulsates with an amplitude of just 0.006 magnitude (6 millimagnitudes), and its pulsation period is 109 seconds.

What ZZ Ceti star has the largest amplitude? As they say, “it’s complicated”. Even though references to a maximum amplitude as high as 0.3^m can be found in the literature, I was unable to find any ZZ Ceti stars with amplitudes greater than 0.12^m. Moreover, pulsation modes of ZZ Ceti stars can come and go, so one observer may observe a higher amplitude but the next may not. Though pulsation modes can and do appear and disappear over time, there is also the changing additive nature of many pulsation modes to consider from one observing run to the next.

Patterson et. al (1991) report m_v=13.0 ZZ Psc having a pulsation amplitude of 0.116^m and period 614.9^s in blue light. Mukadam et al. (2004) report m_v=15.2 UCAC4 448-059643 (in the constellation Serpens) having a pulsation amplitude of 0.121^m and period 873.2^s in blue light.

One challenge in the literature is that pulsation amplitudes are variously given in units of milli-modulation amplitude (mma), milli-magnitudes (mmag), percent, or parts per thousand. Here are the unit conversions:

$1\:mma = \frac{1}{2.5\log_{10}e}\:mmag = 0.1\% = 1\:ppt$

The study of the pulsation modes of white dwarfs and other stars is called asteroseismology. I hope this article has piqued your interest in learning more about this rapidly developing and fascinating field!

References
Bognár, Z., Sódor, Á. 2016, Information Bulletin on Variable Stars,6184
Castanheira, B. G. & Kepler, S. O. 2008, MNRAS, 385, 430
De Gerónimo, F. C., Althaus, L. G., Córsico, et al. 2017, A&A, 599, A21
Dolez, N., Vauclair, G., Kleinman, S. J., et al. 2006, A&A, 446,237
Giammichele, N., Fontaine, G., Bergeron, P., et al. 2015, ApJ, 815, 56
Haro, G., and Luyten, W. J. 1961, Bol. Obs. Tonantzintlay Tacubaya, 3, 35
Kepler, S. O., Robinson, E. L., Koester, D., et al. 2000, ApJ, 539, 379
Kukarkin, B. V., Kholopov, P. N., Kukarkina, N. P., et al. 1972, IBVS, 717, 1
Landolt, A. U. 1968, ApJ, 153, 151
Lasker, B. M. & Hesser, J. E., 1971, ApJ, 163, L89
Lynds, B. T. 1962, ApJS, 7, 1
McGraw J. T. & Robinson E. L., 1976, ApJ, 205, L155
Mukadam, A. S., Mullally, F., Nather, R. E., et al. 2004, ApJ,607, 982
Myers, P. C., Linke, R. A., & Benson, P. J. 1983, ApJ, 264,517
Patterson, J., Zuckerman, B., Becklin E. E., et al. 1991, ApJ, 374, 330

Nova Scuti 2018

Nova Scuti 2018 (or N Sct 2018, for short) was discovered by prolific nova finder Yukio Sakurai of Japan on June 29, 2018. His discovery image at 13:50:36 UT showed the nova shining at magnitude 10.3 (unfiltered CCD magnitude), using only a 180-mm f/2.8 lens plus a Nikon D7100 digital camera. One of his many discoveries is named after him: Sakurai’s Object.

What is a nova? A classical nova is a close binary star system that includes a white dwarf and a “normal” star. The white dwarf siphons material off the other star until a critical density and temperature is reached in the atmosphere of the white dwarf, and a thermonuclear detonation occurs.

Nova Scuti 2018 will eventually receive a variable star designation (V507 Sct?). Here are some typical nova light curves.

Nova Scuti 2018 is located fortuitously close to the 4.7-magnitude star Gamma (γ) Scuti.

Here is a time sequence of images I’ve acquired of Nova Scuti 2018. Comparing with the star chart above, can you find the nova?

Nova Scuti 2018 July 3 6:33:50.632 UT

Nova Scuti 2018 July 4 6:58:42.056 UT

Nova Scuti 2018 July 5 5:41:10.372 UT

Nova Scuti 2018 July 6 5:11:05.779 UT

Nova Scuti 2018 July 7 5:13:05.715 UT

Nova Scuti 2018 July 8 4:38:02.292 UT

Nova Scuti 2018 July 9 5:07:05.015 UT

Nova Scuti 2018 July 10 6:24:01.118 UT

Nova Scuti 2018 July 11 4:48:00.225 UT

Nova Scuti 2018 July 12 4:03:02.246 UT

Nova Scuti 2018 July 17 5:21:03.387 UT

Nova Scuti 2018 July 18 4:13:05.023 UT

Nova Scuti 2018 July 23 5:52:04.507 UT

Nova Scuti 2018 July 24 6:36:01.405 UT

Nova Scuti 2018 July 26 4:31:19.368 UT

Nova Scuti 2018 July 27 4:27:02.468 UT

Nova Scuti 2018 July 28 3:09:03.610 UT

Nova Scuti 2018 July 30 6:01:02.560 UT

Nova Scuti 2018 July 31 4:22:04.683 UT

Nova Scuti 2018 August 3 4:52:03.911 UT

Nova Scuti 2018 August 7 4:08:02.389 UT

Nova Scuti 2018 August 9 5:37:01.365 UT

Nova Scuti 2018 August 10 4:32:05.961 UT

Nova Scuti 2018 August 22 3:16:00.443 UT

Nova Scuti 2018 August 23 2:34:04.551 UT

Nova Scuti 2018 September 12 3:06:01.260 UT

Nova Scuti 2018 September 13 2:58:05.060 UT

Nova Scuti 2018 September 14 4:16:00.450 UT

Milky Way Supernova Candidates

There is a supermassive binary star in our own Milky Way Galaxy that has the potential to create a super-supernova (hypernova?). It could go off tomorrow—or a million years from now. The star system’s name is Eta Carinae. Currently 4th-magnitude and located some 7,500 ly away in the direction of the southern constellation Carina (“The Keel”), Eta Carinae consists of a 100-200 M_☉ star and a 30-80 M_☉ star in a highly-eccentric 5.54^y orbit with the more massive star undergoing prodigious mass loss. Eta Carinae never rises above the horizon unless you’re south of latitude 30° N. So, if Eta Carina ever does go supernova while humans still walk the Earth, you’ll have to travel at least as far as southern Texas or southern Florida to see it. And it will be an impressive sight, easily visible during the daylight hours.

Closer to home, there are seven prime candidates for the next relatively nearby supernova. The nearest of these currently is IK Peg. Keep in mind that over hundreds of thousands of years, stars move quite a lot, so what is close to us now will not necessarily be close to us when a supernova event finally does occur.

IK Pegasi, a binary system comprised of a white dwarf already near the Chandrasekhar limit, and a close-by soon-to-be-giant main-sequence star, lies just 147 to 155 ly away in the direction of the constellation Pegasus, the Winged Horse. IK Peg appears to us visually as a 6th magnitude star located roughly ⅓ of the way from Delphinus to the Square of Pegasus. As the giant star expands into the vicinity of the white dwarf, the white dwarf will accumulate enough material to put it over the Chandrasekhar limit, and a Type Ia supernova will ensue.

Spica (α Vir), located at a distance between 237 and 264 ly, is a massive binary system (10 M_☉ and 7M_☉), with the two stars orbiting each other every four days.

Alpha Lupi (α Lup) is a massive star (~10 M_☉) located between 454 and 476 ly from our solar system.

Antares (α Sco) is a massive star (~12 M_☉, the supernova progenitor) orbited by another massive star (~7 M_☉). However, their orbital period is at least 1,200 years. The Antares system lies between 473 and 667 ly from our solar system

Betelgeuse (α Ori) is a massive star (~12 M_☉) between 500 and 900 ly away. Incidentally, there is a lot of uncertainty about the distance to Betelgeuse, primarily because it’s angular size (44 mas) is an order of magnitude larger than its parallax (4.5 mas) (Harper et al. 2017).

Rigel (β Ori) is a massive star (~23 M_☉) between 792 and 948 ly distant.

Gamma² Velorum (γ² Vel) is a binary system 1,013 to 1,245 ly distant containing two stars which will go supernova in the not-too-distant future. The system consists of a 28.5 M_☉O7.5 giant star and a 9.0 M_☉Wolf-Rayet star (the nearest, incidentally) orbiting each other every 78.5 days. The Wolf-Rayet star will be the first to supernova, followed later by the O giant star.

Tomorrow—or a million years from now? We have no way of accurately predicting. But rest assured, in the unlikely event that any one of these stars goes supernova during our lifetimes, none will be close enough to harm us. Instead, for a time, we will be treated to a object comparable to the Moon in brightness and visible both day and night.

References
Firestone, R.B., 2014, ApJ, 789, 29
Harper, G.M., Brown, A., Guinan, E.F., et al., 2017, AJ, 154, 11
Richardson, N.D., Russell, C.M.P., St-Jean, L., et al., 2017, MNRAS

Brightest Event Ever Observed

On June 14, 2015, perhaps the intrinsically brightest event ever recorded was detected at or near the center of the obscure galaxy APMUKS(BJ) B215839.70−615403.9 in the southern constellation Indus, at a luminosity distance of about 3.8 billion light years.

ASASSN-15lh (All–Sky Automated Survey for SuperNovae), also designated SN 2015L, is located at α₂₀₀₀=22^h02^m15.45^s, δ₂₀₀₀=-61° 39′ 34.6″ and is thought to be a super-luminous supernova—sometimes called a hypernova—but other interpretations are still in play.

Let’s put the brightness of SN 2015L in context. Peaking at an absolute visual magnitude of -24.925 (which would be its apparent visual magnitude at the standard distance of 10 parsecs), SN 2015L would shine as bright as the Sun in our sky if it were 14 light years away—about the distance to van Maanen’s Star, the nearest solitary white dwarf. SN 2015L would be as bright as the full moon if it were at a distance of 8,921 light years. SN 2015L would be as bright as the planet Venus if it were at a distance of 333,000 light years. Since the visible part of our galaxy is only about 100,000 ly across, had this supernova occurred anywhere in our galaxy, it would have been brighter than Venus. If SN 2015L had occurred in M31, the Andromeda Galaxy, 2.5 million light years away, it would take its place (albeit temporarily) as the third brightest star in the night sky (-0.47^m), after Sirius (-1.44^m) and Canopus (-0.62^m), but brighter than Alpha Centauri (-0.27^m) and Arcturus (-0.05^m).

The Open Supernova Catalog (Guillochon et al. 2017) lists three events that were possibly intrinsically brighter than SN 2015L. Two events were afterglows of gamma ray bursts GRB 81007 and GRB 30329: SN 2008hw at -25.014^m and SN 2003dh at -26.823^m, respectively. And the other event was the first supernova detected by the Gaia astrometric spacecraft, Gaia 14aaa, 500 Mly distant, shining perhaps as brightly as -27.1^m.

References
Chatzopoulos E., Wheeler J. C., Vinko J., et al., 2016, ApJ, 828, 94
Dong S., Shappee B. J., Prieto J. L., Jha S. W., et al., 2016, Science, 351, 257
Guillochon J., Parrent J., Kelley L. Z., Margutti R., 2017, ApJ, 835, 64

Two Paths to Low Mass

A brown dwarf (also known as an infrared dwarf) is, in a way, a failed star. Early in their lives, these ultra-low-mass stars (13+ M_J) fuse deuterium into helium-3, and in the highest mass brown dwarfs (65-80 M_J) lithium is depleted into helium-4, as shown below.

But the mass is too low for fusion to be sustained (the temperature and pressure in the core aren’t high enough), and soon the fusion reactions peter out. Then, only the slow process of thermal contraction provides a source of heat for the wanna-be star.

There is another, very different, path to a brown dwarf star. A cataclysmic variable usually consists of a white dwarf and a normal star in a close binary system. As material is pulled off the “donor star” (as the normal star is called) onto the white dwarf, the donor star can eventually lose so much mass that it can no longer sustain fusion in its core, and it becomes a brown dwarf star.

When we see a white dwarf / brown dwarf binary system, how do we know that the brown dwarf wasn’t always a brown dwarf? Strong X-ray and ultraviolet emission provides evidence of an accretion disk around the white dwarf, and astronomers can calculate the rate of mass transfer between the two stars. Often, this is billions of tons per second! Using other techniques to estimate the age of the binary system, we sometimes find that the donor star must have started out as a normal star with much more mass than we see today.

Distant Supernovae Evince Accelerating Expansion of our Universe

In 1998, it was discovered by two independent research teams through the study of distant Type Ia supernovae that our expanding universe has an expansion rate that is accelerating. This was a completely unexpected result.

A Type Ia supernova occurs in a close binary star system where mass from one star accretes onto a white dwarf until it reaches a critical mass and a supernova explosion ensues. Many of these events, chosen carefully, can be used as “standard candles” for distance determination. The intrinsic peak luminosity of a typical Type Ia supernova is a function of the light curve decay time. Type Ia supernovae whose luminosity curves rise and fall more rapidly are less intrinsically luminous at maximum brightness. Type Ia supernovae whose luminosity curves rise and fall more slowly are more intrinsically luminous at maximum brightness.

If we know the intrinsic luminosity of an object (the absolute magnitude) and can measure the apparent luminosity of that object (the apparent magnitude), we can calculate its distance. Type Ia supernovae are on the order of a million times brighter than Cepheid variables, and are in fact the brightest of all “normal” supernovae. They can thus be used to measure the distance to extremely distant objects.

The evidence for an accelerating universe is that these distant supernovae appear fainter than they should be at their measured cosmological redshift, indicating that they are farther away than expected. A number of possible explanations for the faint supernova phenomenon had to be eliminated before the conclusion that the universe’s expansion is accelerating could be arrived at, including

(1) Do distant supernovae (and therefore supernovae that occurred many billions of years ago) have the same intrinsic brightness as comparable nearby supernovae that occurred in the recent past?

(2) Are the distant supernovae being dimmed by galactic and intergalactic extinction due to dust and gas along our line of sight to the supernova?

As described above, the shape of the supernova light curve indicates the supernova’s intrinsic brightness, analogous in a way to the period of a Cepheid indicating its intrinsic brightness. Though there is evidence that ancient supernovae may have been a little different than those today because of lower metallicity, the effect is small and doesn’t change the overall conclusion of an accelerating universe. However, properly characterizing the influence of metallicity will result in less uncertainly in distance and therefore less uncertainty in the acceleration rate of the universe.

Extinction is worse at bluer wavelengths, but how the apparent magnitude changes as a function of distance is independent of wavelength, so the two effects can be disentangled. 2011 Nobel physics laureate Adam Riess in his award-winning 1996 Ph.D. thesis developed a “Multicolor Light Curve Shape Method” to analyze the light curves of a large ensemble of type Ia supernovae, both near and far, allowing him to determine their distances more accurately by removing the effects of extinction.

Eridanus Delights

The sixth largest constellation in the sky stretches from near Rigel on the west side of Orion down to 1st-magnitude lucida Achernar (declination -57°), a star that rotates so rapidly that its polar diameter is not even ¾ its equatorial diameter (Domiciano de Souza et al. 2014). Achernar (α Eri) is appropriately named. It means “The End of the River” in Arabic.

Eridanus, the River, contains two very special, easily seen, stars. 40 Eridani (also known as Keid and Omicron² Eridani), a visual triple star system (magnitudes 4.4, 9.5, and 11.2) just 16.3 light years away, presents the most easily observed white dwarf star, 9.5-magnitude 40 Eri B, visible in any telescope.

A little further west we can find 3.7-magnitude Epsilon Eridani, the nearest star beyond the Alpha Centauri system thought to harbor one or more planets. Compared to our Sun, ε Eri is cooler (K2V), much younger (200-800 Myr), and somewhat metal-deficient (74% solar), and it is just 10.5 light years away. This youthful star still sports a dusty disk between radii 35 and 75 AU (Greaves et al. 1998), inside of which its putative planet, Epsilon Eridani b—at least 0.6 to 0.9 Jupiter masses—travels around the star in a highly elliptical orbit, completing one revolution every 6.85 to 7.26 years. At periastron, Epsilon Eridani b lies between 1.0 and 2.1 AU from its parent star, and at apastron, its distance is 4.9 to 5.8 AU (Mizuki et al. 2016). However, the existence of this or any other planets in the system is still far from certain, primarily due to the high level of photospheric activity that is difficult to disentangle from the radial velocity signals of any possible orbiting planets (Giguere et al. 2016).

References
Domiciano de Souza, A., Kervella, P., et al. 2014, A&A, 569, A10
Giguere, M. J., Fischer, D. A., et al. 2016, ApJ, 824, 150
Greaves, J. S., Holland, W. S., et al. 1998, ApJL, 506, L133
Mizuki, T., Yamada, T., et al. 2016, A&A, 595, A79