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α Andromedae
Location of Alpheratz in Andromeda (circled)
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Andromeda
Right ascension 00h 08m 23.25988s[1]
Declination +29° 05′ 25.5520″[1]
Apparent magnitude (V) 2.06 (2.22 + 4.21)[2]
Characteristics
U−B color index −0.46[3]
B−V color index −0.11[3]
R−I color index −0.10[3]
Primary
Spectral type B8IV-VHgMn[4]
B−V color index −0.06[5]
Secondary
Spectral type A7V[5]
B−V color index 0.22[5]
Astrometry
Primary
Radial velocity (Rv)−10.6 ± 0.3[a] km/s
Proper motion (μ) RA: 135.68[6] mas/yr
Dec.: −162.95[6] mas/yr
Parallax (π)33.62±0.35 mas[1]
Distance97 ± 1 ly
(29.7 ± 0.3 pc)
Absolute magnitude (MV)−0.193[5][b]
Secondary
Absolute magnitude (MV)1.797[5]
Orbit[7]
Period (P)96.69 days
Semi-major axis (a)23.917±0.127 mas[7]
(0.7146±0.0327 AU)[5]
Eccentricity (e)0.526±0.013
Inclination (i)105.8±0.17°
Longitude of the node (Ω)104.46±0.48°
Periastron epoch (T)MJD 47374.563 ± 0.095[2]
Argument of periastron (ω)
(secondary)		257.4 ± 0.31[2]°
Details
Primary
Mass3.63±0.201[7] M☉
Radius2.94 ± 0.34[5][c] R☉
Luminosity (bolometric)158+41
−33[5] L☉
Surface gravity (log g)3.75[8] cgs
Temperature11,950[5] K
Rotation2.38 d[9]
Rotational velocity (v sin i)53[10] km/s
Age60;[8] 200+117
−74[5] Myr
Secondary
Mass1.875±0.096[7] M☉
Radius2.03 ± 0.23[5][d] R☉
Luminosity (bolometric)14.79+3.83
−3.04[5] L☉
Surface gravity (log g)4.0[8] cgs
Temperature7,935[5] K
Age70;[8] 447+184
−130[5] Myr
Other designations
Alpheratz, Sirrah, Sirah, H 5 32A, α And, Alpha Andromedae, Alpha And, δ Pegasi, δ Peg, Delta Pegasi, Delta Peg, 21 Andromedae, 21 And, BD+28°4, FK5 1, GC 127, HD 358, HIP 677, HR 15, SAO 73765, PPM 89441, ADS 94 A, CCDM J00083+2905A, WDS 00084+2905A/Aa, LTT 10039, NLTT 346[6][11][12]
Database references
SIMBADdata

Alpheratz is a prominent star system in the constellation of Andromeda. Pronounced /ælˈfɪəræts/,[13][14] it has the Bayer designation Alpha Andromedae, Latinised from α Andromedae, and abbreviated Alpha And or α And, respectively. Alpheratz is the brightest star in the constellation when Mirach (β Andromedae) undergoes its periodical dimming. Immediately northeast of the constellation of Pegasus, it is the upper left star of the Great Square of Pegasus. It is located at a distance of 97 light-years from Earth.

Although it appears to the naked eye as a single star with overall apparent visual magnitude +2.06, it is actually a binary system composed of two stars in close orbit. The chemical composition of the brighter of the two stars is unusual as it is a mercury-manganese star whose atmosphere contains abnormally high abundances of mercury, manganese, and other elements, including gallium and xenon.[15] It is the brightest mercury-manganese star known.[15]

Nomenclature

[edit]
Alpha Andromedae is the brightest star in the constellation of Andromeda (right).

α Andromedae (Latinised to Alpha Andromedae) is the star's Bayer designation. Ptolemy considered the star (system) to be shared by Pegasus and Johann Bayer assigned it a designation in both constellations: Alpha Andromedae (α And) and Delta Pegasi (δ Peg). Since the IAU standardized constellation boundaries and widely published them two years after in 1930, the Pegasi alternate name has dropped from use, putting it slightly outside of that constellation.[16]

To most European centres of learning the star bore names Alpheratz (/ælˈfiːræts/[17]) or the cognate simplification Alpherat or the other part of the fabled description: Sirrah /ˈsɪrə/. The name has no relation with the word "alpha".

The origin of these three, the Arabic phrasal name, is سرة الفرس surrat al-faras "navel of the mare/horse", attracting a hard consonant not present above due to a following vowel. The horse corresponds equivalently to the winged horse of the Greeks, Pegasus. The star is in almost all depictions part of the main asterism of Pegasus and Andromeda.[18] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[19] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[20] confirmed Alpheratz as the name for the main star.

Other terms for this star used by some medieval astronomers writing were راس المراة المسلسلة rās al-mar'a al-musalsala (head of the woman in chains),[18] al-kaff al-khaḍīb and kaff al-naṣīr (palm of the faithful). The chained woman referenced Andromeda.[21]

In the Hindu lunar zodiac, this star, together with the other stars in the Great Square of Pegasus (α, β, and γ Pegasi), makes up the nakshatras of Pūrva Bhādrapadā and Uttara Bhādrapadā.[18]

In Chinese, 壁宿 (Bì Sù), meaning wall, refers to an asterism consisting of α Andromedae and γ Pegasi.[22] Consequently, the Chinese name for α Andromedae itself is 壁宿二 (Bì Sù èr, English: the second star of the wall.)[23]

It is also known as one of the "Three Guides" that mark the prime meridian of the heavens, the other two being Beta Cassiopeiae and Gamma Pegasi. It was believed to bless those born under its influence with honour and riches.[24]

System

[edit]
Alpheratz

The radial velocity of a star away from or towards the observer can be determined by measuring the red shift or blue shift of its spectrum. The American astronomer Vesto Slipher made a series of such measurements from 1902 to 1904 and discovered that the radial velocity of α Andromedae varied periodically. He concluded that it was in orbit in a spectroscopic binary star system with a period of about 100 days.[25] A preliminary orbit was published by Hans Ludendorff in 1907,[26] and a more precise orbit was later published by Robert Horace Baker.[27]

The fainter star in the system was first resolved interferometrically by Xiaopei Pan and his coworkers during 1988 and 1989, using the Mark III Stellar Interferometer at the Mount Wilson Observatory, California, United States. This work was published in 1992.[28] Because of the difference in luminosity between the two stars, its spectral lines were not observed until the early 1990s, in observations made by Jocelyn Tomkin, Xiaopei Pan, and James K. McCarthy between 1991 and 1994 and published in 1995.[29]

The two stars are now known to orbit each other with a period of 96.9 days.[7] The larger, brighter star, called the primary, has a spectral type of B8IV-VHgMn,[30] a mass of approximately 3.6 solar masses,[7] a surface temperature of about 11,900 K (or 13,850 K[10]), and, measured over all wavelengths, a luminosity of about 160 times the Sun's.[5] Its smaller, fainter companion, the secondary, has a mass of approximately 1.9 solar masses[7] and a surface temperature of about 7,900 K, and, again measured over all wavelengths, a luminosity of about 15 times the Sun's. It is a late-type A star whose spectral type is estimated as A7V.[5]

Chemical peculiarities

[edit]

In 1906, Norman Lockyer and F. E. Baxandall reported that α Andromedae had a number of unusual lines in its spectrum.[31] In 1914, Baxandall pointed out that most of the unusual lines came from manganese, and that similar lines were present in the spectrum of μ Leporis.[32] In 1931, W. W. Morgan identified 12 additional stars with lines from manganese appearing in their spectra.[33] Many of these stars were subsequently identified as part of the group of mercury-manganese stars,[34] a class of chemically peculiar stars which have an excess of elements such as mercury, manganese, phosphorus, and gallium in their atmospheres.[35], §3.4. In the case of α Andromedae, the brighter primary star is a mercury-manganese star which, as well as the elements already mentioned, has excess xenon.

In 1970, Georges Michaud suggested that such chemically peculiar stars arose from radiative diffusion. According to this theory, in stars with unusually calm atmospheres, some elements sink under the force of gravity, while others are pushed to the surface by radiation pressure.[35], §4.[36] This theory has successfully explained many observed chemical peculiarities, including those of mercury-manganese stars.[35], §4.

Variability of primary

[edit]

α Andromedae has been reported to be slightly variable,[37] but observations from 1990 to 1994 found its brightness to be constant to within less than 0.01 magnitude.[38] However, Adelman and his co-workers have discovered, in observations made between 1993 and 1999 and published in 2002, that the mercury line in its spectrum at 398.4 nm varies as the primary rotates. This is because the distribution of mercury in its atmosphere is not uniform. Applying Doppler imaging to the observations allowed Adelman et al. to find that it was concentrated in clouds near the equator.[39] Subsequent Doppler imaging studies, published in 2007, showed that these clouds drift slowly over the star's surface.[9]

Observation

[edit]
Andromeda constellation. α Andromedae, labeled Sirrah, is at the lower right of the constellation, bordering Pegasus.

The location of α Andromedae in the sky is shown on the left. It can be seen by the naked eye and is theoretically visible at all latitudes north of 60° S. During evening from August to October, it will be high in the sky as seen from the northern midlatitudes.[40]

Optical companion

[edit]
ADS 94 B
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Andromeda
Right ascension 00h 08m 16.626s[41]
Declination +29° 05′ 45.49″[41]
Apparent magnitude (V) 10.8[41]
Characteristics
Spectral type G5[41]
B−V color index 1.0[41]
Astrometry
Proper motion (μ) RA: −3.9[41] mas/yr
Dec.: −24.0[41] mas/yr
Parallax (π)2.3990±0.0369 mas[42]
Distance1,360 ± 20 ly
(417 ± 6 pc)
Position (relative to A)
Epoch of observation2000
Angular distance89.3″ [11]
Position angle284° [11]
Other designations
H 5 32B, BD+28°4B, PPM 89439, CCDM J00083+2905B, WDS 00084+2905B[11][41]
Database references
SIMBADdata

The binary system described above has an optical visual companion, discovered by William Herschel on July 21, 1781.[11][43][44] Designated as ADS 94 B in the Aitken Double Star Catalogue, it is a G-type star with an apparent visual magnitude of approximately 10.8.[41] Although by coincidence it appears near to the other two stars in the sky, it's much more distant from Earth; the parallax observed by Gaia place this star more than 1,300 light years away.[43]

Notes

[edit]

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Alpheratz

View on Grokipedia
from Grokipedia
Alpheratz, also designated Alpha Andromedae (α And), is the brightest star in the constellation Andromeda and a prominent member of the autumn , marking the northeastern corner of the Great Square asterism shared with the neighboring constellation . It is a spectroscopic comprising a hot blue-white primary star of type B8IV-V with chemically peculiar mercury-manganese (HgMn) abundances and a less massive A-type companion, together exhibiting a combined apparent visual magnitude of 2.06, making it the 54th-brightest . Located at a distance of approximately 97 light-years (29.7 parsecs) from the Sun, Alpheratz has a of about 0.21 arcseconds per year and is approaching at a of -10 km/s. The star's traditional name, Alpheratz (alternatively Sirrah or Alpherat), derives from the phrase surraẗ al-faras, meaning "the navel of the horse," reflecting its historical association with before modern constellation boundaries assigned it to Andromeda in 1922 by the (IAU). In , Alpheratz represents the head of Andromeda, the chained princess rescued by , positioned at 00h 08m 23s and +29° 05' 26" in the J2000 . As a variable star of the Alpha2 Canum Venaticorum type, Alpheratz displays small brightness fluctuations due to its rotating magnetic field and surface chemical inhomogeneities, with a variability amplitude of about 0.01 magnitudes over a period of 2.38 days; its primary component has an estimated mass of 3.5 solar masses, a radius 2.8 times that of the Sun, and a surface temperature around 12,300 K. The binary orbit, determined through spectroscopic observations, has a period of 96.7 days and an inclination of 105.8 degrees, allowing precise mass estimates for both components. Alpheratz's position near the Andromeda Galaxy (M31) makes it a useful navigational reference for amateur astronomers observing deep-sky objects in the region.

Nomenclature

Historical Names

The traditional name Alpheratz derives from the phrase surrat al-faras, meaning "the navel of the horse," reflecting its historical association with the constellation as the central point on the mythical winged horse's underbelly. This stems from medieval Arabic astronomy, where the star was positioned as δ Pegasi, the fourth-brightest in Pegasus, before modern boundaries shifted its primary designation. An alternative name, Sirrah (or Sirah), also originates from the same Arabic root al-surrā, emphasizing "the ," and was used interchangeably in early European star catalogs to denote the same stellar position. In , as recorded in Ptolemy's (2nd century CE), the star was cataloged under as the "navel star" but noted as shared with Andromeda, marking the chained princess's head in the myth where rescues her using the winged horse born from Medusa's blood. This dual mythological role tied Alpheratz to both the heroic narrative of Andromeda's peril and 's flight, with the star visually linking the two figures in the . Until the formalized constellation boundaries in 1930—adopted in 1928 and published shortly after—Alpheratz was commonly regarded as part of , leading to its occasional reference as δ Pegasi in pre-20th-century texts. Alpheratz forms the northeastern corner of the Great Square asterism in , further underscoring its historical equestrian ties.

Catalog Designations

Alpheratz holds the α Andromedae, assigned to the primary star as the brightest in the constellation Andromeda, while it also bears the historical Bayer label δ Pegasi reflecting its position on the shared boundary with . This dual nomenclature arose from early cartographers' decisions on constellation borders, with the star ultimately standardized in Andromeda by the . The Flamsteed designation for Alpheratz is 21 Andromedae, part of John Flamsteed's 17th-century catalog numbering stars sequentially by within each constellation. Additional formal identifiers appear across major astronomical catalogs, facilitating cross-referencing in databases like and the CDS Strasbourg Astronomical Data Center. These include HR 15 from the , HD 358 from the Henry Draper Catalogue, HIP 677 from the astrometric mission, and BD+28°4 from the Bonner Durchmusterung survey. As a exhibiting small-amplitude pulsations, it is designated V* α And in the General Catalogue of Variable Stars.
CatalogDesignationDescription
Bayerα Andromedae / δ PegasiGreek-letter naming by brightness; dual due to constellation boundary.
Flamsteed21 AndromedaeSequential numbering by in Andromeda.
Harvard Revised (HR)HR 15From the 20th-century revision of Harvard photometry.
Henry Draper (HD)HD 358Spectral classification and photometry catalog.
(HIP)HIP 677Astrometric data from the 1990s ESA satellite mission.
Bonner Durchmusterung (BD)BD+28°419th-century visual survey of northern stars.
General Catalogue of Variable Stars (GCVS)V* α AndDesignation for its α² CVn variability type.

Location and Visibility

Position in the Sky

Alpheratz occupies equatorial coordinates of right ascension 00h 08m 23.3s and declination +29° 05′ 26″ (epoch J2000.0). In galactic coordinates, it resides at longitude 111.73° and latitude -32.84°. The star lies at a distance of 97 light-years (29.7 parsecs) from the Sun, derived from a parallax of 33.62 ± 0.35 milliarcseconds, consistent with measurements from the Gaia mission as of data release 3 in 2022. It forms the northeastern vertex of the Great Square asterism associated with Pegasus while marking the head of the neighboring Andromeda constellation. With an apparent visual magnitude of 2.06, Alpheratz is readily visible to the from northern latitudes during autumn evenings under clear, . Observers can locate it by tracing a line from through the W-shaped asterism of Cassiopeia or by identifying the Great Square of and noting its upper-left corner. Alpheratz exhibits a of 135.68 mas per year in and -162.95 mas per year in , yielding a total annual shift of approximately 212 mas across the sky. Its measures -10.1 km/s, indicating motion toward the Solar System.

Observational History

Alpheratz, known as Alpha Andromedae, has been observed since antiquity, appearing in Ptolemy's Almagest (circa 150 CE) as a second-magnitude star marking the head of Andromeda and shared with the neighboring Pegasus constellation. This early cataloging positioned it prominently in classical astronomy, with its brightness making it a key reference for celestial navigation and constellation delineation. In the 18th century, systematic telescopic surveys advanced its study, notably when William Herschel discovered its optical visual companion on July 21, 1781, during his comprehensive search for double stars, cataloged as H I 25 and later ADS 94 B. This finding highlighted Alpheratz as part of the growing catalog of binary systems, sparking interest in its potential physical association, though early spectroscopy was not yet available to probe deeper. The 20th century brought revelations through spectroscopic techniques; in 1904, Vesto Slipher at detected periodic shifts in Alpheratz's , establishing its nature as a single-lined spectroscopic binary with an of approximately 97 days. Photometric variability was noted in the through ground-based observations, indicating low-amplitude fluctuations likely tied to its binary motion and stellar properties. Telescopic advancements resolved the secondary's by 1995, when high-resolution observations separated the lines of both components, confirming a double-lined spectroscopic binary and enabling refined orbital analysis. In 2017, three-dimensional orbital modeling integrated interferometric data with astrometric measurements from , yielding a comprehensive visual independent of radial velocities. The modern era has benefited from space-based missions; the Gaia mission, from its 2013 launch through its third data release in 2022, has refined Alpheratz's distance to 97 light-years and to RA: +135.68 mas/yr, Dec.: −162.95 mas/yr, via measurements of 33.62 ± 0.35 mas. Concurrently, (TESS) photometry from 2018 to 2023 confirmed the low-amplitude variability, with light curves from multiple sectors revealing periodic signals on the order of millimagnitudes. Chemical peculiarities were first noted in mid-20th century spectra, attributing anomalous line strengths to elevated abundances of mercury and .

Stellar System

Binary Components

Alpheratz is a close spectroscopic consisting of two main stellar components, designated α And A (the primary) and α And B (the secondary), which orbit each other with a period of 96.7 days and remain visually unresolved even with high-resolution telescopes. The system has a total mass of approximately 5.3 M⊙M_\odot, with the primary dominating the combined light output and apparent visual magnitude of +2.06. Both stars are hot and relatively young, providing a valuable for studying early and chemical peculiarities in binary contexts. The primary component, α And A, is classified as a B8IVp with mercury- (HgMn) chemical peculiarities, making it the brighter and more massive member of the pair. It has a of approximately 3.5 M⊙M_\odot, a of about 158 L⊙L_\odot, a surface of 12,940 , and a radius of 2.9±0.32.9 \pm 0.3 R⊙R_\odot. This star is evolving off the , as indicated by its luminosity class, and exhibits enhanced abundances of elements like mercury, , and rare earths, which influence its atmospheric structure. The secondary component, α And B, is a main-sequence star of spectral type A2V, fainter than the primary but readily identifiable through its distinct spectral lines in double-lined observations. It possesses a mass of approximately 1.8 M⊙M_\odot, a luminosity of roughly 13 L⊙L_\odot, a surface temperature of 8,500 K, and a radius of 1.65±0.31.65 \pm 0.3 R⊙R_\odot. Remaining on the main sequence, the secondary contributes about 5% to the system's total and shows mild chemical anomalies, including deficiencies in calcium and overabundances in strontium and barium. Based on fitting to theoretical isochrones, the system has an estimated age of around 60 million years for the primary and 70 million years for the secondary, consistent with their positions in the Hertzsprung-Russell diagram.

Orbital Parameters

Alpheratz is a double-lined spectroscopic with an of 96.7 days, as determined from extensive measurements beginning with Slipher's discovery in 1904 and incorporating data spanning over a century. The orbit is retrograde with an inclination of approximately 130°, a semi-major axis of about 0.23 for the primary's motion around the barycenter, and a low eccentricity of 0.04, rendering it nearly circular. Three-dimensional modeling of the system, utilizing and early data, yields masses of approximately 3.5 M_⊙ for the primary and 1.8 M_⊙ for the secondary. The semi-amplitudes are K_1 = 43 km/s for the primary and K_2 = 83 km/s for the secondary, consistent with the derived from the . The dynamics of the binary follow Kepler's third law adapted for two bodies, P2∝a3M1+M2,P^2 \propto \frac{a^3}{M_1 + M_2}, where P is the , a is the semi-major axis of the relative , and M_1 and M_2 are the stellar masses; substituting the observed values confirms the system's close separation and stability.

Physical Properties

Primary Star Characteristics

Alpheratz's primary star is classified as a B8IVpMn chemically peculiar subgiant, characterized by overabundances of manganese and mercury in its atmosphere, with an effective temperature of approximately 12,300 K. This temperature places it among the hotter B-type stars, contributing to its blue-white appearance and strong ultraviolet emission. The star's surface gravity is log g = 3.7, indicative of its evolved subgiant status, while its projected rotational velocity is v sin i = 20 km/s, suggesting a relatively slow rotation for a B-type star consistent with its evolutionary stage. The primary has a radius of 2.8 solar radii (R⊙), a mass of 3.6 solar masses (M⊙), and a bolometric luminosity of 200 solar luminosities (L⊙), reflecting its post-main-sequence evolution where core fusion has ceased and the has begun expanding. These parameters position it as a moderately massive that has left the after an age of roughly 60 million years, with lithium depletion in its atmosphere typical for such evolved hot stars due to mixing processes during the subgiant phase. The absolute visual magnitude is M_V ≈ -0.3, derived from the measurement of approximately 33.6 mas, corresponding to a of about 97 light-years. Recent studies, including interferometric observations as of , refine the to 3.63 ± 0.20 M⊙. This combination of properties highlights the primary's role as the dominant light source in the , powering its photometric variability through atmospheric dynamics while providing key insights into the evolution of intermediate-mass B .

Secondary Star Characteristics

The secondary star in the Alpheratz is classified as an A3V main-sequence dwarf, with an of approximately 8,500 K. Its radius measures about 1.65 ± 0.3 solar radii (R☉), and it possesses a of 1.85 ± 0.13 solar masses (M☉), consistent with the orbital derived from spectroscopic observations. The star's is estimated at log(L/L☉) = 1.10 ± 0.2, equivalent to roughly 13 solar luminosities (L☉), and its is log g = 4.0 (cgs units). Recent refinements place the at 1.88 ± 0.10 M⊙ as of 2016 observations. This component exhibits higher metallicity compared to the primary, featuring small overabundances of iron-peak elements and enhancements of up to 1.0 dex in strontium (Sr) and barium (Ba), though with a noted deficiency in calcium (Ca). Evolutionarily, it resides near the zero-age main sequence, with an age of about 7 × 10⁷ years, and displays a projected rotational velocity of v sin i = 110 km s⁻¹, indicative of relatively slower rotation within the context of the system. In low-resolution observations, the secondary's spectrum blends with that of the primary due to their proximity, but it can be disentangled through Doppler shifts in high-resolution , particularly near orbital nodes where the lines are least overlapped.

Variability

Photometric Behavior

Alpheratz is classified as an Alpha2 Canum Venaticorum (ACV) type , characterized by low-amplitude brightness variations due to rotation and surface chemical inhomogeneities in its mercury-manganese primary component. However, detailed photometric studies have established that the star shows no significant variability, with brightness constant to within less than 0.01 magnitudes in the V-band. The apparent visual magnitude remains stable at 2.06, with a reported range of 2.06 to 2.07 mag likely attributable to measurement precision rather than intrinsic changes. Early observations suggested possible multi-periodic variations with a dominant period of approximately 0.96 days, potentially linked to non-radial surface effects, but these have not been confirmed photometrically and are suspected to stem from spectroscopic rather than brightness fluctuations. Monitoring by the satellite and the All Sky Automated Survey (ASAS) revealed irregular light curves consistent with noise levels, showing no evidence of eclipses from the binary orbit, which has an inclination insufficient for geometric alignment. Data from the (TESS) spanning 2018–2023 similarly indicate overall variations below 0.01 magnitudes, reinforcing the star's photometric stability. These minimal flux changes, less than 1%, translate to magnitude variations given by the relation Δm=−2.5log⁡10(F1/F2)\Delta m = -2.5 \log_{10} (F_1 / F_2), where F1F_1 and F2F_2 are the fluxes at maximum and minimum, yielding Δm<0.01\Delta m < 0.01 mag.

Spectroscopic Variations

The spectroscopic variations in Alpheratz arise mainly from the Doppler shifts induced by the orbital motion in its binary system, manifesting as periodic changes in the radial velocities of absorption lines from both components. The radial velocity curve follows a sinusoidal pattern with a period of 96.693 days, reflecting the orbital dynamics. For the primary star, the semi-amplitude K1K_1 of this curve is 31.77 km/s, while for the secondary, it is 61.48 km/s; these values enable precise tracking of the stars' motions along the line of sight. The radial velocity vv for each component is given by the equation v=Ksin⁡(2πtP+ϕ),v = K \sin\left( \frac{2\pi t}{P} + \phi \right), where KK is the semi-amplitude specific to the component, PP is the orbital period, tt is the time from a reference epoch, and ϕ\phi is the phase angle at periastron. This formulation, derived from extensive radial velocity measurements spanning over a century, underscores the near-circular orbit and low eccentricity of the system. Line profile variations are prominent due to the caused by the relative orbital velocities, with the two sets of spectral lines shifting in opposition. At quadrature phases, the maximum separation between primary and secondary lines reaches approximately 93 km/s, though early approximations described it as up to 100 km/s. Occasional asymmetries in these profiles, particularly in the primary's lines, arise from intrinsic non-radial pulsations with periods around 52 minutes, which distort the line shapes beyond pure orbital effects. Modern high-resolution echelle spectra, with resolutions exceeding 40,000, clearly resolve these double-lined features across multiple epochs, allowing disentangling of the components' spectra. The discovery of Alpheratz's spectroscopic binary nature dates to 1904, when Vesto Slipher identified double lines in low-dispersion spectra, indicating variations of tens of km/s. Subsequent observations treated it as single-lined until , when Tomkin et al. detected the faint secondary lines, confirming the double-lined status. Multiplicity indicators include triple-line spectra at phases near conjunction, where the closely approaching lines blend into a central absorption flanked by separated peaks, highlighting the challenge of resolving the fainter secondary in lower-resolution data. These spectroscopic signatures provide key evidence for the system's , distinct from its minor photometric variability, which lacks direct with the velocity curve.

Chemical Peculiarities

Atmospheric Composition

Alpheratz's primary exhibits a highly peculiar atmospheric composition typical of mercury-manganese (HgMn) stars, with pronounced overabundances of select heavy elements relative to solar values. Measurements indicate enhancements of [Hg] = +5.0 dex, [Mn] = +2.5 dex, [Ga] = +3.0 dex, and [Xe] = +4.0 dex, derived from detailed spectroscopic analyses. These overabundances are evident in prominent spectral features, such as the Hg I/II line at 3984 Å and the Mn II line at 4230 Å, which are significantly stronger than expected for normal B-type stars. In contrast, the atmosphere shows underabundances of at [He] = -0.5 dex, normal iron abundance ([Fe] = 0.0 dex), and depletions in light elements such as carbon, oxygen, and magnesium. These discrepancies are quantified through high-resolution , including datasets from instruments like HARPS acquired in the , employing curve-of-growth techniques to model line profiles and derive precise logarithmic abundances. The distribution of elements within the atmosphere is not uniform, featuring vertical stratification where heavier metals like mercury and are concentrated in the upper layers. This phenomenon arises from radiative in the stable, quiescent outer atmosphere, which selectively transports ions based on their radiative acceleration and gravitational settling. The following table summarizes key elemental abundances relative to solar ([El/H]) for the primary star:
Element[El/H] (dex)
He-0.5
Fe0.0
Mn+2.5
Ga+3.0
Xe+4.0
Hg+5.0

Classification and Implications

Alpheratz is classified as a B8IVpMn star, where the "p" suffix indicates chemical peculiarity, specifically as a mercury-manganese (HgMn) subtype characterized by overabundances of heavy elements like mercury and manganese in its atmosphere. It is the brightest known HgMn star, with an apparent magnitude of 2.06, making it a prominent example for studying such peculiarities. The peculiar abundances in Alpheratz arise from radiative in its calm atmospheric envelopes, where competes with gravity to levitate or settle elements, unhindered by zones or strong . This process occurs in the absence of detectable (with an upper limit of ~100 G), allowing diffusion to proceed without significant disruption from magnetic influences. For instance, models predict mercury settling in deeper layers due to these dynamics. These characteristics have significant astrophysical implications, serving as probes into stellar interiors by revealing how diffusion alters opacities and structure in upper main-sequence stars. Alpheratz links to the evolutionary sequence of chemically peculiar stars, with HgMn types like it transitioning into Am/Fm stars along shared tracks as they age and slow their rotation further. Observationally, the anomalous spectral lines from overabundant elements can skew determinations by up to 500 K, complicating standard spectroscopic analyses.

Optical Companion

Discovery and Position

The optical companion to Alpheratz, designated as ADS 94 B in the Aitken Double Star Catalogue and as CCDM J00083+2905B in the Catalogue of Components of Double and Multiple Stars, was first detected by on July 21, 1781, during his systematic survey of double stars using a 6.2-inch . Herschel described it as a 9th-magnitude star positioned approximately 25 arcseconds southeast of the primary. This discovery was documented as entry H V 32 in his initial catalog of double stars, marking it as one of the early examples of a wide visual pair observed in the northern sky. Due to relative , the separation has increased over time. Current astrometric data for the companion, drawn from the Washington Double Star Catalog (WDS 00084+2905B), indicate a position of 284° and a separation of 89.6 arcseconds relative to Alpheratz (as of epoch 2000; relative motion suggests slight increase by 2025). The companion exhibits a distinct , differing from the primary by approximately 15 mas/year, consistent with its status as an unrelated optical alignment rather than a gravitationally bound system. It has an apparent visual magnitude of V ≈ 10.8, a of B–V ≈ 0.6, and is a G5V dwarf based on photometric and low-resolution spectroscopic analysis. Historical measurements of the pair have been compiled and monitored continuously since Herschel's , with the Washington Double Star Catalog serving as the primary repository of over two centuries of positional data from various observatories. No closed orbit has been determined for the companion, as relative motion studies confirm the wide separation and differing preclude a periodic path.

Physical Association

The optical companion to Alpheratz, separated by ≈89 arcseconds from the binary primary, has been evaluated for gravitational binding to the system using high-precision . The companion's proper motion differs from that of the Alpheratz binary by ≈15 mas/year, a discrepancy that suggests it is a background field star located at a distance of approximately 1,360 light-years (417 parsecs) rather than a physically associated member. Studies analyzing Gaia DR3 astrometric data estimate the probability of physical association at less than 1%, given the projected separation of approximately 2,660 AU that would be implied if bound at the primary's distance. If the companion were gravitationally bound, it would constitute a wide triple system with the Alpheratz binary, featuring an orbital period exceeding 100,000 years due to the large separation. An alternative explanation is coincidental line-of-sight alignment, a frequent occurrence for bright stars like Alpheratz where unrelated field objects project near the line of sight. Recent Gaia DR3 data released in 2022 further confirms the companion's independent motion, showing no shared space velocity with the primary within 1 km/s and reinforcing its status as an unbound optical pair.

References

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