Stars/X-ray classification/Laboratory

This laboratory is an activity for you to classify a star of your choice using its X-ray characteristics. While it is part of the astronomy course principles of radiation astronomy, it is also independent.

Some suggested classification entities to consider are wavelength range, periodicity, spectrum, mass, binarity, Euclidean space, Non-Euclidean space, or spacetime.

More importantly, there are your classification entities. Search SIMBAD Astronomical Database for your star to be sure it has been detected as an X-ray source.

You may choose to define your X-ray classification entities or use those already available.

Usually, research follows someone else's ideas of how to do something. But, in this laboratory you can create these too.

Okay, this is an astronomy X-ray classification of a star laboratory, but you may create what an X-ray classification of a star is.

Yes, this laboratory is structured.

I will provide an example of an X-ray classification of a star. The rest is up to you.

Questions, if any, are best placed on the discussion page.

Notations
You are free to create your own notation or use those already provided.

Control groups
For creating an X-ray classification of your star, what would make an acceptable control group? Think about a control group to compare your classification of your star or your process of classifying it too.

One likely control group is the visual classification of stars.

Candidate stars
According to SIMBAD, Vega (alpha Lyrae) (delta Sct type variable) is an X-ray source in the first Einstein catalog (1E) and an ultraviolet source from the CEL, EUVE, and TD1 catalogs. It has not been detected as a gamma-ray source.

At one time (1932), Vega was listed as a double star. Also in 1963, Vega is listed as a visual double star. Again in 1983, it was listed as a double star. This was repeated in 1994. It is still listed as a double star in 1996. The update in 2001 also lists it. The star with Vega is 56.41 arcsec away and is designated as BD+38 3238D of unknown spectral class. That these two stars are undifferentiated between double star or binary star for some 70 years at only 25 lyrs away is remarkable.

The images at the top of the resource have a 24-micron image in blue on the left and a 70-micron infrared image on the right. "The [debris] disc extends to at least 815 astronomical units." "The images are 3 arcminutes on each side." BD+38 3238D is unspecified or unlocated within the infrared images but should be within the debris disc. According to SIMBAD there are ten objects within one arcminute of Vega including BD+38 3238D, a submillimeter source (JCMTSE J183656.4+384709), and eight infrared sources: [ MHW2003] 1-8.

X-ray analysis
The X-ray "counts observed from [...] Vega in the HRI are very likely to be due entirely to UV contamination from the photospheric emission, and hence the X-ray luminosities for [...] Vega [...] should be replaced by upper limits at least one order of magnitude lower, i.e., log LX, Vega < 26.6 [...] Thus the HRI observation of Vega is completely consistent with the upper limit obtained in the IPC, and the only remaining inconsistency concerning the X-ray emission from Vega is the rocket experiment by Topka et al. (1979), who report a detection of Vega in a 5 s pointing yielding 7 counts; however, in our opinion these authors do not convincingly rule out the possibility of UV contamination. Note in this context that the IPC's used for Topka et al.s' (1979) rocket flight and for the Einstein Observatory were not identical."

"Many types of main sequence stars emit in the X-ray portion of the spectra. In massive stars, strong stellar winds ripping through the extended atmosphere of the star create X-ray photons. On lower mass stars, magnetic fields twisting through the photosphere heat it sufficiently to produce X-rays. But between these two mechanisms, in the late B to mid A classes of stars, neither of these mechanisms should be sufficient to produce X-rays. Yet when X-ray telescopes examined these stars, many were found to produce X-rays just the same."

"The first exploration into the X-ray emission of this class of stars was the Einstein Observatory, launched in 1978 and deorbited in 1982. While the telescope confirmed that these B and A stars had significantly less X-ray emission overall, seven of the 35 A type stars still had some emission. Four of these were confirmed as being in binary systems in which the secondary stars could be the source of the emission, leaving three of seven with unaccounted for X-rays."

"The German ROSAT satellite found similar results, detecting 232 X-ray stars in this range. Studies explored connections with irregularities in the spectra of these stars and rotational velocities, but found no correlation with either. The suspicion was that these stars simply hid undetected, lower mass companions."

Either "the main star truly is the source, or there are even more elusive, sub-arcsecond binaries skewing the data."

On July 27, 1977, at 05:41:48.1 UTC, an Aerobee 350 or boosted Black Brant launched from White Sands Missile Range using Vega as a reference by its star tracker to update its position while maneuvering between X-ray targets, automatically observing Vega with its X-ray telescope for 4.8 s [graphed in the image on the right].

The quantity of detected photons (7) in the band 0.2-0.80 keV corresponds to an X-ray luminosity LX ≈ 3 x 1028 erg s-1.

"The ANS 3 σ upper limit for Vega (2.5 x 1028 ergs s-1) is only slightly lower than our flux measurement."

"Because the X-ray [luminosity] of Vega [is] much closer to that of the Sun than to the typical galactic X-ray sources which have been detected to date, it is natural to consider processes analogous to solar coronal activity as the explanation for the X-ray activity."

"Vega is thought to be a solitary star, and therefore noncoronal X-ray-producing mechanisms seem to be excluded".

"Vega is the first solitary main-sequence star beyond the Sun known to be an X-ray emitter".

Vega's "computed X-ray surface luminosity [...] is comparable to that of the quiet Sun [...] Note, however, that because of our very short exposure, the average level of coronal emission may vary significantly from our single measurement."

"Using estimates of the stellar [radius] derived from stelar structure calculations, we obtain [a] surface X-ray [luminosity] of ~6.4 x 104 ergs cm-2 s-1 for Vega [that falls] within the range of solar coronal X-ray emission, which can vary between ~8 x 103 ergs cm-2 s-1 in coronal holes and ~3 x 106 ergs cm-2 s-1 in active regions".

Magnetic "field activity, leading to coronal heating, may account for Vega's X-ray emission because of inhomogeneous distribution of surface magnetic flux and associated coronal activity."

That Vega is regarded as an X-ray source rests on one 4.8 s star-tracking observation by one sounding rocket flight carrying an X-ray detector flown on many flights that yields trustworthy results.

"Vega is a pole-on, highly oblate, rapid rotator [...] the star exhibits extreme limb darkening and a large decrement in effective temperature from pole to equator. [...] the best fittingmodel (Teff pole = 10150 K, Teff eq = 7900 K, θ = 3.329 mas) has the pole inclined 5° to line of sight and rotates at 91% of the angular speed of break-up, resulting in a temperature drop of 2250 K from center to limb. [...] the total luminosity [...] is emitted in a highly non-homogeneous manner with five times more UV flux being emitted from the pole as is emitted in the equatorial plane, while the visible through near-IR flux is some 70% greater at the pole than that of the equatorial plane and 54% greater than that expected from a slow or non-rotating A0 V star."

A 'polar coronal hole' is a coronal hole that occurs above one or both rotational poles of a star that has a coronal cloud around it.

"The radiant emission from coronal holes is greatly diminished relative to other coronal regions".

The "emission is proportional to the integral of the square of the electron density along the line of sight [...] Data of this type are therefore heavily influenced by regions of high density along the line of sight--the low corona for disk observations, and denser structures surrounding coronal holes for limb observations."

An "analysis of the northern polar region during the period 1973 June 29 to July 13 [...] can be summarized as follows. The boundary of the hole is essentially axisymmetrc about the polar axis and is nearly radial from 3 to 6 R⊙. The boundary at these heights is located at 25° ± 5° latitude, although it is of much smaller extent (boundary ~65° latitude) as observed near the solar surface with the American Science and Engineering (AS&E) X-ray experiment on Skylab [...] the increase of the polar hole's cross sectional area from the surface to 3 R⊙ is approximately 7 times greater than for a purely radial boundary."

For α Lyr, log FX/FV = -6.79 (variable X-ray source), log LX 27.6 erg s-1 (variable X-ray source). Upper limits were log FX/FV = -7.4 and log LX 27.0 erg s-1.

A coronal cloud is not a diffuse, homogeneous hot atmosphere, but one or more strongly structured, topologically closed features dominated by magnetic confinement.

Magnetic field of Vega
An extensive convection zone is not required, and any star with magnetic field strengths and geometry similar to the Sun's will possess a corona.

Magnetic fields on the order of ~30 gauss have been reported for Vega (~1 gauss for the Sun), so perhaps these substantially higher average field strengths compensate for the expected reduced convective activity, resulting in surface X-ray luminosities comparable to the quiet Sun.

Using spectropolarimetry, a magnetic field has been detected on the surface of Vega by a team of astronomers at the Observatoire du Pic du Midi. They "report the detection of a magnetic field on Vega and argue that Vega is probably the first member of a new class of yet undetected magnetic A-type stars."

The "polarization [is] a Zeeman signature [that] leads to a value of [Bl =] -0.6 ± 0.3 G for the disk-averaged line-of-sight component of the surface magnetic field."

"The strength of Vega magnetic field is about 50 micro-tesla, which is close to that of the mean field on Earth and on the Sun."

Rotational velocity of Vega
"Vega rotates in less than a day, while the Sun's rotation period is 27 days."

Infrared analysis
"The infrared excesses are well modeled by two components, a warm belt close to the star, and a cooler belt farther out. The clear separation of the belts could be explained by the presence of planets clearing the gap."

The graph at left shows the clear separation of infrared belts for Vega. This separation may "be explained by the presence of planets clearing the gap."

Sun as an X-ray source
The photosphere of the Sun does not emit X-rays. The chromosphere and the transition region either emit ultraviolet, extreme ultraviolet, or X-rays (most likely soft X-rays). The coronal clouds around the Sun emit X-rays and sometimes gamma-rays, neutrinos, neutrals, protons, positrons, and electrons, among other solar cosmic-rays.

Report
An X-ray classification of the star Vega (α Lyrae)

by Marshallsumter (discuss • contribs) 02:16, 7 April 2014 (UTC)

Abstract

The X-ray characteristics of the Sun have been studied since the 1940s. The Sun may be considered an X-ray variable star as its intensity corresponds to the sunspot cycle. This cycle may have its origins in the mechanisms that heat the photosphere and the coronal clouds around the Sun. An analysis of the X-ray characteristics of Vega suggest that it is a Sun-like X-ray star.

Introduction

It has taken a number of decades to observe and describe the X-ray properties of Vega. Associated with these properties are a magnetic field comparable to the Sun and the possibility of a coronal hole over the pole facing Earth.

Experiment

By scanning the available literature to ascertain the X-ray properties of Vega, several apparent conclusions may be drawn. The X-ray output of Vega falls between that of the Sun. The X-ray output of the Sun minimizes during the solar cycle quiet period and maximizes at the sunspot maximum.

Vega is considered an X-ray variable star with a corona and an X-ray output comparable to the Sun.

Results

Vega is considered to be Sun-like in its X-ray output and variability. The peak of X-ray output may not be directly observable as the star has a pole facing Earth.

Discussion

The photosphere of Vega has a diameter five times that of the Sun. Its apparent stellar system has similarities to the solar system but is scaled as approximately four times larger. Even with a pole temperature above 10,000 K Vega produces no detectable X-ray output from its photosphere, just like the Sun at 5,778 K. Vega is a visual spectral type A0V star which means it is transparent. The Sun is not considered transparent at visual wavelengths.

The physical size of the photosphere of Vega and its transparency may reflect more the cloud it formed from than anything else.

The above surface fusion that is occurring on the Sun may not be happening above the photosphere of Vega as no flares or gamma rays have been detected.

Conclusion

Vega is a Sun-like X-ray source.

Evaluation
To assess your X-ray classification of the star of your choice, including your justification, analysis, results, and discussion, I will provide such an assessment of my examples for comparison.

Evaluation

Even the visual classification of stars uses more than just visual characteristics general to the star. It uses spectral lines of specific elements, size, shape, metallicity, and special elemental compositions. Classifying Vega solely on the basis of its apparent X-ray characteristics and leaving the other properties to its cloud of origin seems short-sighted.

Hypotheses

 * 1) Vega is very much like the Sun from the point of view of X-ray astronomy.