An Am star or metallic-line star is a type of chemically peculiar star of spectral type A whose spectrum has strong and often variable absorption lines of metals such as zinc, strontium, zirconium, and barium, and deficiencies of others, such as calcium and scandium. The original definition of an Am star was one in which the star shows "an apparent surface underabundance of Ca (and/or Sc) and/or an apparent overabundance of the Fe group and heavier elements". [1]
The unusual relative abundances cause the spectral type assessed from the Calcium K lines to be systematically earlier than one assessed from other metallic lines. Typically, a spectral type judged solely from hydrogen lines is intermediate. This leads to two or three spectral types being given. For example, Sirius has been given a spectral type of kA0hA0VmA1, indicating that it is A0 when judged by the Calcium k line, A0V when judged by its hydrogen lines, and A1 when judged by the lines of heavy metals. [2] There are other formats, such as A0mA1Va, again for Sirius. [3] [4]
The chemical abnormalities are due to some elements which absorb more light being pushed towards the surface, while others sink under the force of gravity. This effect takes place only if the star has low rotational velocity. [5] Normally, A-type stars rotate quickly. Most Am stars form part of a binary system in which the rotation of the stars has been slowed by tidal braking. [5]
The best-known metallic-line star is Sirius (α Canis Majoris). The following table lists some metallic-line stars in order of descending apparent visual magnitude.
δ Delphini and ρ Puppis
A small number of Am stars show unusually late spectral types and particularly strong luminosity effects. Although Am stars in general show abnormal luminosity effects, stars such as ρ Puppis are believed to be more evolved and more luminous than most Am stars, lying above the main sequence. Am stars and δ Scuti variables lie in approximately the same location on the H–R diagram, but it is rare for a star to be both an Am star and a δ Scuti variable. ρ Puppis is one example and δ Delphini is another. [2]
Several authors have referred to a class of stars known as δ Delphini stars, Am stars but with relatively little difference between the calcium and other metallic lines. They have also been compared to the δ Scuti stars. Later studies showed that the group was somewhat inhomogeneous, possibly coincidental, and recommended dropping use of the δ Delphini class in favour of a narrower class of ρ Puppis stars with relatively high luminosity and late spectral types. [2] [11] However, there is still sometimes confusion, for example with ρ Puppis stars being considered to all be δ Scuti variables. [12]
Delphinus is a small constellation in the Northern Celestial Hemisphere, close to the celestial equator. Its name is the Latin version for the Greek word for dolphin (δελφίς). It is one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and remains one of the 88 modern constellations recognized by the International Astronomical Union. It is one of the smaller constellations, ranked 69th in size. Delphinus' five brightest stars form a distinctive asterism symbolizing a dolphin with four stars representing the body and one the tail. It is bordered by Vulpecula, Sagitta, Aquila, Aquarius, Equuleus and Pegasus.
A variable star is a star whose brightness as seen from Earth changes with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as either:
Delta Capricorni, or δ Capricorni, is a binary star located 38,7 light-years from the Sun in the constellation of Capricornus. The system consists of an eclipsing binary, Delta Capricorni A, and two visual companions that are over 10 magnitudes fainter, labeled B and C. Delta Capricorni A's two components are designated Delta Capricorni Aa and Ab. The primary star, Aa, is a white giant and the combined light of Aa and Ab makes it the brightest star in the constellation.
Delta Scuti, Latinized from δ Scuti, is a variable star in the southern constellation Scutum. With an apparent visual magnitude that fluctuates around 4.72, it is the fifth-brightest star in this small and otherwise undistinguished constellation. Analysis of the parallax measurements place this star at a distance of about 199 light-years from Earth. It is drifting closer with a radial velocity of −45 km/s.
Theta Tucanae is a star in the constellation Tucana.
Tau Ursae Majoris (τ UMa) is the Bayer designation for a binary star in the northern circumpolar constellation of Ursa Major. It is visible to the naked eye, having an apparent visual magnitude of 4.66. With an annual parallax shift of 25.82 mas, it is located about 126 light years from the Sun. At that distance, the visual magnitude is diminished by an extinction factor of 0.19 due to interstellar dust.
IK Pegasi is a binary star system in the constellation Pegasus. It is just luminous enough to be seen with the unaided eye, at a distance of about 154 light years from the Solar System.
EW Aquarii, or HR 8102, is a variable star in the equatorial constellation of Aquarius. With an apparent visual magnitude that fluctuates around 6.47, it is a dim star near the lower limit of visibility to the naked eye. The star is located at a distance of approximately 439 light years from the Sun based on parallax, but is drifting closer with a radial velocity of −39 km/s. The star is positioned near the ecliptic and thus is subject to lunar occultations. It may be a member of the Hyades Group.
k Puppis is a Bayer designation given to an optical double star in the constellation Puppis, the two components being k1 Puppis and k2 Puppis.
HD 30453 is a binary star system in the northern constellation of Auriga. It is visible to the naked eye with an apparent visual magnitude of 5.86. The system is located at a distance of approximately 334 light years from the Sun based on parallax. It is drifting further away from the Sun with a radial velocity of 16.65 km/s.
V352 Aurigae is a variable star in the northern constellation of Auriga. It dimly visible to the naked eye with an apparent visual magnitude that ranges from 6.13 down to 6.18. According to the Bortle scale, it is faintly visible to the naked eye from dark rural skies. The star is located at a distance of approximately 970 light years from the Sun based on parallax, but is drifting closer with a radial velocity of −7 km/s.
Delta Delphini, Latinized from δ Delphini, is a binary star in the northern constellation of Delphinus. It is visible to the naked eye with an apparent visual magnitude of 4.43. Based upon an annual parallax shift of 14.61 mas as seen from the Earth, the system is located about 223 light years from the Sun.
Rho Puppis, formally named Tureis, is a star in the southern constellation of Puppis. With an average apparent visual magnitude of 2.78, it is the third-brightest member of this generally faint constellation. Based upon parallax measurements made during the Hipparcos mission, Rho Puppis is located at a distance of 63.5 light-years from the Sun. It is the prototype of the ρ Puppis class of evolved Am stars.
SX Phoenicis is a variable star in the southern constellation Phoenix. With an apparent visual magnitude ranging around 7.33, it is too faint to be readily seen with the naked eye and requires binoculars. It is located 272 light years from the Sun, as determined from an annual parallax shift of 12 mas.
V529 Andromedae, also known as HD 8801, is a variable star in the constellation of Andromeda. It has a 13th magnitude visual companion star 15" away, which is just a distant star on the same line of sight.
Rho Pavonis, Latinized from ρ Pavonis, is a single, variable star in the southern constellation of Pavo. It is yellow-white in hue and faintly visible to the naked eye with an apparent visual magnitude that fluctuates around 4.86. The star is located at a distance of approximately 190 light years from the Sun based on parallax, and is drifting further away with a radial velocity of +8 km/s. It is a candidate outlying member of the Tucana Association of co-moving stars.
AZ Canis Minoris is a variable star in the equatorial constellation of Canis Minor. It is just visible to the naked eye in good viewing conditions as a dim, white-hued star with an apparent visual magnitude of around 6.46. The star is located around 500 light years away from the Sun based on parallax, and is drifting further away with a radial velocity of +15 km/s. No evidence has been found for a companion to this star, although in the past it has been reported as a binary star system.
AZ Phoenicis is a variable star in the constellation of Phoenix. It has an average visual apparent magnitude of 6.47, so it is at the limit of naked eye visibility. From parallax measurements by the Gaia spacecraft, it is located at a distance of 322 light-years from Earth. Its absolute magnitude is calculated at 1.65.
17 Comae Berenices is a multiple star system in the northern constellation of Coma Berenices. The brighter component, 17 Com A, is a naked eye star with an apparent visual magnitude of 5.2. It has a faint companion of magnitude 6.6, 17 Com B, positioned at an angular separation of 146.4″ along a position angle of 251°, as of 2018. They are located at a distance of approximately 240 light years from the Sun based on parallax measurements.
HD 40372, also known as 59 Orionis, V1004 Orionis and HR 2100, is a variable star in the constellation Orion. Its apparent magnitude varies between magnitude 5.88 and 5.92, making it faintly visible to the naked eye for an observer far from light polluted urban areas. HD 40372 exhibits two types of variability; it is an eclipsing binary star and one of the two stars is a Delta Scuti variable star.
