# Stars/Dwarfs

A dwarf star is a star of relatively small size and low luminosity, including the majority of main sequence stars.

## Ultraviolet subdwarfs

The subdwarf B star is a kind of subdwarf star with spectral type B. They differ from the typical subdwarf star by being much hotter and brighter. They are from the "extreme horizontal branch stars" of the Hertzsprung–Russell diagram.
Subdwarf B stars, being more luminous than white dwarfs, are a significant component in the hot star population of old stellar systems, such as globular clusters, spiral galaxy bulges and elliptical galaxies. They are prominent on ultraviolet images. The hot subdwarfs are proposed to be the cause of the UV-upturn in the light output of elliptical galaxies.

## US 708

"Scientists using the W. M. Keck Observatory and Pan-STARRS1 telescopes on Hawaii have discovered a star that breaks the galactic speed record, traveling with a velocity of about 2.7 million mph (1,200 km/s). This velocity is so high, the star will escape the gravity of our galaxy. In contrast to the other known unbound stars, the team showed that this compact star was ejected from an extremely tight binary by a thermonuclear supernova explosion."
"US 708 has another peculiar property in marked contrast to other hypervelocity stars: It is a rapidly rotating, compact helium star likely formed by interaction with a close companion. Thus, US 708 could have originally resided in an ultra-compact binary system, transferring helium to a massive white dwarf companion, ultimately triggering a thermonuclear explosion of a type Ia supernova. In this scenario, the surviving companion, US 708, was violently ejected from the disrupted binary as a result and is now traveling with extreme velocity."

## White dwarfs

A white dwarf is a small very dense star that is typically the size of a planet. A white dwarf is formed when a low-mass star has exhausted all its central nuclear fuel and lost its outer layers as a planetary nebula.

## Theoretical white dwarfs

Def. a "dying star of low or medium mass, more solid [and dense] but less bright than the sun" is called a white dwarf.

## Accretions

If a white dwarf has a close companion star that overflows its Roche lobe, the white dwarf will steadily accrete gas from the companion's outer atmosphere. The companion may be a main sequence star, or one that is aging and expanding into a red giant. The captured gases consist primarily of hydrogen and helium, the two principal constituents of ordinary matter in the universe. The gases are compacted on the white dwarf's surface by its intense gravity, compressed and heated to very high temperatures as additional material is drawn in. The white dwarf consists of degenerate matter, and so does not inflate at increased heat, while the accreted hydrogen is compressed upon the surface. The dependence of the hydrogen fusion rate on temperature and pressure means that it is only when it is compressed and heated at the surface of the white dwarf to a temperature of some 20 million kelvin that a nuclear fusion reaction occurs; at these temperatures, hydrogen burns via the CNO cycle.
While hydrogen fusion can occur in a stable manner on the surface of the white dwarf for a narrow range of accretion rates, for most binary system parameters the hydrogen burning is thermally unstable and rapidly converts a large amount of the hydrogen into other heavier elements in a runaway reaction, liberating an enormous amount of energy, blowing the remaining gases away from the white dwarf's surface and producing an extremely bright outburst of light. The rise to peak brightness can be very rapid or gradual which is related to the speed class of the nova; after the peak, the brightness declines steadily. The time taken for a nova to decay by 2 or 3 magnitudes from maximum optical brightness is used to classify a nova via its speed class. A fast nova will typically take less than 25 days to decay by 2 magnitudes and a slow nova will take over 80 days.
"An accreting white dwarf undergoes [near surface] nuclear burning when the accretion rate exceeds a certain limit." Due to the near surface nuclear burning, "the stellar luminosity is dominated by hydrogen burning, since the energy liberated by hydrogen burning exceeds that due to accretion on a white dwarf by an order of magnitude or more, depending on the mass of the white dwarf."
"[A]bove an accretion rate (with a hydrogen abundance of 0.7 by mass) M$_{RG}$ ≈ 8.5 10$^{-7}$ (M$_{WD}$/M$_ʘ$ -0.52)M$_ʘ$ yr$^{-1}$ (M$_{WD}$=mass of the white dwarf) the accreted matter forms a red-giant like envelope around the white dwarf, with the luminosity being generated from hydrogen shell burning."

## Single white dwarfs

The youngest, hottest WD is very close to 100,000 K, of type DO and is the first single WD recorded as an X-ray source with ROSAT.

## PG 1159 stars

PG 1159 stars are a group of very hot, often pulsating WDs for which the prototype is PG 1159 dominated by carbon and oxygen in their atmospheres.
PG 1159 stars reach luminosities of ~10$^{38}$ erg/s but form a rather distinct class. RX J0122.9-7521 has been identified as a galactic PG 1159 star.

## Yellow subdwarfs

Yellow subdwarfs are in luminosity class VI. "[Y]ellow high-velocity subdwarfs are easily confused with white dwarfs in a proper-motion selection."
HD 64090 is a color class G0 subdwarf.

## Brown dwarfs

Brown dwarfs are sub-stellar objects that have fully convective surfaces and interiors, with no chemical differentiation by depth. Brown dwarfs occupy the mass range between that of large gas giant planets and the lowest-mass stars; this upper limit is between 75$^{[1]}$ and 80 Jupiter masses ($M_{J}$).
Astronomers have reported that spectral class T brown dwarves (the ones with the coolest temperatures) are colored magenta because of absorption by sodium and potassium atoms of light in the green portion of the spectrum.
"The star on the left is so much brighter than the "coolest star" that it creates the white streak and dramatic pattern visible in the image."

## Theoretical brown dwarfs

A brown dwarf is a celestial object intermediate in size between a giant planet and a small star, believed to emit mainly infrared radiation.
Def. a "starlike object that contracts to about the volume of the planet Jupiter after its formation phase; its mass may range from several times that of Jupiter, such that it fuses deuterium, up to just below the threshold of sustained hydrogen fusion" is called a brown dwarf.

## 2MASS J10475385+2124234

"2MASS J10475385+2124234 [is] a brown dwarf more than 33 light-years away in the constellation Leo. The dwarf ... has a surface temperature of just ... 900 Kelvin ... Jupiter's lights are linked to its rapid rotation ... Since brown dwarfs are comparable in size to Jupiter, the brown dwarf flare mechanisms might arise similarly. ... [When] first examined using the ... fixed radio dish at Arecibo Observatory in Puerto Rico In several observations, ... flares of radio activity [occurred] ... [Using the] Karl G. Jansky Very Large Array (VLA) of telescopes ... The radio waves emanating ... are about 4.5 times fainter than the previous record ... observing ... LPP 944-20."

## DH Tauri's companion

"DH Tauri's companion [...] is a brown dwarf with only 40 times the mass of Jupiter. DH Tauri is a young star only one million years old in the constellation Taurus. It is so young it will not begin nuclear fusion for another one hundred million years. It is 460 light years away and two thirds as massive as the Sun. It's companion is among the coolest and lightest of known brown dwarfs orbiting young stars. If the companion had been less massive it probably would have been a planet."
"Planets weigh less than 13 times the mass of Jupiter. Brown dwarfs are 13 to 80 times more massive than Jupiter."
"An image of the star DH Tauri (abbreviated as DH Tau) [contained] an object 250 times fainter 2.3 arcseconds away. At the distance of DH Tauri (460 light years), this separation is equivalent to 330 [AU]. Although the object was in older images of DH Tauri, its location in the new image revealed that it was not an unrelated background object, but a companion that orbits DH Tauri."
The "surface temperature of the companion is about 2700 to 2800 degrees Kelvin, its surface gravity is 4 times that of Jupiter and its mass is only 40 times larger than Jupiter. This puts the companion in the brown dwarf category."

## Sub-brown dwarfs

A sub-brown dwarf is an astronomical object of planetary mass that is not orbiting a star and is not considered to be a brown dwarf because its mass is below the limiting mass ... [of] about 13 Jupiter masses). ... Sub-brown dwarfs are formed in the manner of stars, through the collapse of a gas cloud (perhaps with the help of photo-erosion), and not through accretion or core collapse from a circumstellar disc [although] not universally agreed upon; astronomers are divided into two camps as whether to consider the formation process of a planet as part of its division in classification. ... The smallest mass of gas cloud that could collapse to form a sub-brown dwarf is about 1 M$_J$. This is because to collapse by gravitational contraction requires radiating away energy as heat and this is limited by the opacity of the gas.

## Red dwarfs

A red dwarf is a small and relatively cool star on the main sequence, of either late K or M spectral type. Red dwarfs are by far the most common type of star in the Milky Way, at least in the neighborhood of the Sun. Proxima Centauri, the nearest star to the Sun, is a red dwarf. Due to their low luminosity, individual red dwarfs cannot easily be observed. From Earth, none are visible to the naked eye.
Typical characteristics :
| Stellar Class | Mass (M$_ʘ$) | Radius (R$_ʘ$) | Luminosity (L$_ʘ$) | T$_{eff}$ (K) |
| M0V | 60% | 62% | 7.2% | 3,800 |
| M1V | 49% | 49% | 3.5% | 3,600 |
| M2V | 44% | 44% | 2.3% | 3,400 |
| M3V | 36% | 39% | 1.5% | 3,250 |
| M4V | 20% | 26% | 0.55% | 3,100 |
| M5V | 14% | 20% | 0.22% | 2,800 |
| M6V | 10% | 15% | 0.09% | 2,600 |
| M7V | 9% | 12% | 0.05% | 2,500 |
| M8V | 8% | 11% | 0.03% | 2,400 |
| M9V | 7.5% | 8% | 0.015% | 2,300 |
"[O]ut [of] a sample of 3,897 red dwarfs ... [the Kepler Space Telescope ]has identified 95 exoplanet candidates circling them. Three of these candidates are roughly Earth-size and orbit within their stars' "Goldilocks zone," where liquid water (and possibly life as we know it) can exist."

## Proxima Centauri

Proxima Centauri is a red dwarf star about 4.22 light-years (4.0×10$^{13}$ km) distant in the constellation of Centaurus.
Proxima Centauri has a radius of 98,100 ± 4,900 km. Its surface effective temperature is 3,042 ± 117 K.
Although it has a very low average luminosity, Proxima is a flare star that undergoes random dramatic increases in brightness because of magnetic activity.
More than 85% of its radiated power is at infrared wavelengths.
The resulting flare activity generates a total X-ray emission similar to that produced by the Sun.
In 1951, American astronomer Harlow Shapley announced that Proxima Centauri is a flare star. Examination of past photographic records showed that the star displayed a measurable increase in magnitude on about 8% of the images, making it the most active flare star then known.
The proximity of the star allows for detailed observation of its flare activity. In 1980, the Einstein Observatory produced a detailed X-ray energy curve of a stellar flare on Proxima Centauri. Further observations of flare activity were made with the EXOSAT and ROSAT satellites, and the X-ray emissions of smaller, solar-like flares were observed by the Japanese ASCA satellite in 1995. Proxima Centauri has since been the subject of study by most X-ray observatories, including XMM-Newton and Chandra.
These flares can grow as large as the star and reach temperatures measured as high as 27 million K—hot enough to radiate X-rays. Indeed, the quiescent X-ray luminosity of this star, approximately (4–16) x 10$^{26}$ erg/s ((4–16) x 10$^{19}$ W), is roughly equal to that of the much larger Sun. The peak X-ray luminosity of the largest flares can reach 10$^{28}$ erg/s (10$^{21}$ W.)
The chromosphere of this star is active, and its spectrum displays a strong emission line of singly ionized magnesium at a wavelength of 280 nm. About 88% of the surface of Proxima Centauri may be active, a percentage that is much higher than that of the Sun even at the peak of the solar cycle. Even during quiescent periods with few or no flares, this activity increases the corona temperature of Proxima Centauri to 3.5 million K, compared to the 2 million K of the Sun's corona. However, the overall activity level of this star is considered low compared to other M-class dwarfs,
Proxima Centauri has a relatively weak stellar wind, resulting in no more than 20% of the Sun's mass loss rate from the solar wind.
"[S]imultaneous Einstein and IUE observations of the dM5e flare star Proxima Centauri ... On 1979 March 6 ... [and] ... Again on 1980 August 20 [captured] X-ray [flares] on Proxima [Centauri] ... The X-ray characteristics of this flare event are strongly suggestive of a solar analog: the two-ribbon flare. ... [This] less common flare class, the long decay X-ray events, are characterized by large and diffuse systems of X-ray loops and are associated with prominence eruptions generally away from active regions."

## AZ Cancri

At right is a close-up of the SDSS DR6 image of AZ Cancri in real (visual) color. According to SIMBAD, AZ Cancri (AZ Cnc) is a spectral type M6.0V flare star, that is also an X-ray source detected by the ROSAT satellite. AZ Cancri (AZ Cnc) is a M-type flare star in the constellation Cancer. It has an apparent visual magnitude of approximately 17.59.

### Astrometry

The star is in NGC 2632 designated Haro, Chavira, and Gonzalez (HCG) 4. NGC 2632 is an open cluster, also called Messier 44, and the Praesepe Cluster.
The X-ray astronomy satellite ROSAT detected AZ Cnc at RX J0840.4+1824 and 1RXS J084029.9+182417.
In the SIMBAD visual sky plot at right, AZ Cnc is immediately at the tip of the red arrow J2000.0 RA 08$^h$ 40$^m$ 29.751$^s$ Dec +18°24'09.18".
Regarding the NED image at left AZ Cnc is located at J2000.0 RA 08$^h$ 40$^m$ 30.2$^s$ Dec +18°23'55", precisely coincident with the negative visual object image at exact image center.

### Astronomical visual sources

The visual star is spectral type M6e, specifically M6.5Ve. Visual magnitude is M$_v$ = 16.9. log L$_{bol}$ = 30.48 ergs/s (or 3.020 x 10$^{30}$ ergs s$^{-1}$).
A comparison of images between that from SIMBAD at upper right and the NASA/IPAC Extragalactic Database (NED) (5' x 5') at upper left shows some identical visual object patterns. In the NED image at left are two objects very close together. These two are near the center of the southeast quarter of the SIMBAD image. To the east-southeast of the NED designated AZ Cnc is an apparently solitary visual object that is also in the NED image. In the north-east quadrant of the NED image are four visual objects forming a triangle with a weak source just above two of the objects. This pattern is repeated on the SIMBAD image which includes a fifth object forming a diamond with the objects of the triangle.

### Physical characteristics

AZ Cnc is considered a very low mass star (VLMS). Distance from the Sun is 14.0 pc. The radial velocity of LHS 2034 is 64.2±0.6 km/s. Its galactic motion (space velocities) are U = -60.6 km/s, V = -44.3 km/s, and W = -8.3 km/s, relative to the local standard of rest. LHS 2034 belongs kinematically to the old disk. Its rotational velocity is v sin(i) = 7.9±2.8 km/s.

### Catalog designations

CSI+18-08377 is the Catalog of Stellar Identifications.
GJ 316.1 is the catalog entry from the nearby star data published between 1969-1978 for numbers 2001-2159 and incorporating the earlier Catalogue of nearby stars by Gliese.
LHS 2034 is the Luyten-H-S Catalogue number.

### Astronomical X-ray sources

The X-ray luminosity log (L$_x$) = 27.40 ergs/s (or 2.512 x 10$^{27}$ ergs s$^{-1}$). L$_x$/L$_{bol}$ = 8.318 x 10$^{-4}$ does not depend on M$_v$, at least for older stars like AZ Cnc.

### Flarings

The X-ray luminosity of AZ Cnc increased by at least two orders of magnitude during a flare that lasted more than 3 h and reached a peak emission level of more than 10$^{29}$ ergs/s. During another long duration flare (March 14, 2002) on LHS 2034, very strong wing asymmetries occurred in all lines of the Balmer series and all strong He I lines, but not in the metal lines. LHS 2034 was observed for 1.5 h on March 14, 2002, and 40 m on March 16, 2002.
The flaring atmosphere of LHS 2034 has been modeled with the PHOENIX atmosphere code, consisting of
* an underlying photosphere,
* a linear temperature rise vs. log column mass in the chromosphere, and
* transition region (TR) with different gradients.
For the underlying photosphere, T$_{eff}$ = 2800 K, log g = 5.0, and a solar chemical composition was used. The last spectrum taken in the series after the flare was used for the quiescent chromosphere.
The line asymmetries have been attributed to downward moving material, specifically a series of flare-triggered downward moving chromospheric condensations, or chromospheric downward condensations (CDC)s as on the Sun. For the Sun such events can last a few minutes, but for LHS 2034 they have lasted for 1.5 h.

### Theory of coronal heating

The electrodynamic coupling theory of coronal heating developed in a solar context, has been applied to stellar coronae. A distinctive feature is the occurrence of a resonance between the convective turnover time and the crossing time for Alfvén waves in a coronal loop. The resonance attains a maximum among the early M dwarf spectral types and declines thereafter. A turnover in coronal heating efficiency, presumably manifested by a decrease in L$_x$/L$_{bol}$, becomes evident toward the late M spectral types when the theory is applicable. This is consistent with an apparent lack of X-ray emission among the late M dwarfs. Coronal heating efficiencies do not decrease toward the presumably totally convective stars near the end of the main sequence. For "saturated" M dwarfs, 0.1% of all energy is typically radiated in X-rays, while for AZ Cnc this number increases during flaring to 7%. So far there is no evidence to suggest that AZ Cnc is less efficient than more massive dwarfs in creating a corona. The saturation boundary in X-ray luminosity extends to late M dwarfs, with L$_x$/L$_{bol}$ ~ 10$^{−3}$ for saturated dwarfs outside flaring. No coronal dividing line exists in the Hertzsprung–Russell diagram at the low-mass end of the main sequence.
AZ Cnc casts doubt on the applicability of electrodynamic coupling as there is no evidence for a sharp drop in L$_x$/L$_{bol}$ when compared with other late M stars at least until subtype M8.

### Dynamos

AZ Cnc has a corona and this may indicate that a distributive dynamo is just as efficient in producing magnetic flux as a shell dynamo. Between the generation of a magnetic field and the emission of X-rays lies the coronal heating mechanism.

## Gliese 623

"This Hubble Space Telescope picture resolves, for the first time, one of the smallest stars in our galaxy. Called Gliese 623b or Gl 623b, the diminutive star (right of center) is ten times less massive than the Sun and 60, 000 times fainter."

## Theoretical red dwarfs

Def. a "small, relatively cool star of the main sequence; most stars in the Milky Way are red dwarfs" is called a red dwarf.