proper motions of stars in the region of the great nebula in orion
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ISSN 1063-7729, Astronomy Reports, 2011, Vol. 55, No. 2, pp. 123–131. c© Pleiades Publishing, Ltd., 2011.Original Russian Text c© S.V. Vereshchagin, N.V. Chupina, 2011, published in Astronomicheskii Zhurnal, 2011, Vol. 88, No. 2, pp. 143–151.
Proper Motions of Stars in the Region of the Great Nebula in Orion
S. V. Vereshchagin and N. V. ChupinaInstitute of Astronomy, Russian Academy of Sciences, Moscow, Russia
Received June 8, 2010; in final form, July 1, 2010
Abstract—Six Tautenburg astrograph plates have been used to derive proper motions for 12 740 stars inthe region of the Great Nebula in Orion. The Oricat catalog of proper motions and B and R photometryhas been compiled, incorporating as well data from other published catalogs. The proper motions presentedin different catalogs are compared. The Oricat catalog is useful for studies of features of the structure andkinematics of star clusters and groups.
DOI: 10.1134/S1063772910121029
1. INTRODUCTION
The Orion Nebula (M42) is a densely populatedGalactic star-forming region. The mean distancebetween stars is only 0.05 pc [1]. This has attractedthe attention of many researchers to this region. Ofspecial interest is the young Trapezium cluster, inwhose vicinity are located several more clusters andstellar sub-groups (NGC 1981, NGC 1977, OMC-2,NGC 1980), whose origin is not clear: they maybe open star clusters or sub-clusters in the coronaof the Trapezium [2]. Massive proper-motion andphotometric measurements were obtained earlier byParenago [3] (to B0 ≈ 18m for more than 2000 stars).Several hundred stars are considered in a number ofother studies. A survey of results is presented in [4].
We have conducted mass determinations of propermotions using photographic observations acquiredon the Tautenburg astrograph. This has yielded theOricat catalog for a region 3◦ × 3◦ in size centeredon α = 83.45◦, δ = −5.20◦, which includes 12 740stars to Blim = 18.5m. We have compared the datain our catalog with the catalogs of [4, 5], the PPMXcatalog, and the Hipparcos (HIP) catalog in both itsoriginal [6] and revised [7] versions. The final form ofOricat is supplemented with the data of Parenago [3],and includes 13 577 stars.
2. OBSERVATIONAL MATERIAL
We measured six photographic plates obtained onthe 2-m Schmidt reflector of the Tautenburg Obser-vatory. The largest difference between the epochs was36 years. Table 1 presents information about theseplates: the plate number, date it was obtained, emul-sion, filter, center coordinates, and observer. Eachplate covers a 3◦ × 3◦ region of sky, with a scale of51.4 ′′/mm (the plates are 210× 210 mm in size). Part
of the area on each of these plates includes emissionfrom the Great Nebula in Orion, which is about 1◦in size and is located between the eastern edge andcenter of the plates, leading to a loss in the number ofstars.
The photographic plate images were translatedinto electronic form using the Tautenburg Observa-tory plate scanner [8]. The accuracy of the scanningwas 0.5 μm, and the scale was 1000 pixels/mm.The rectangular coordinates x, y and instrumentalmagnitudes were then determined using a specializedprogram [9].
3. DETERMINATION OF THE RA AND DECOF THE MEASURED STARS
We adopted reference stars for determining theequatorial coordinates from the ACT catalog [10],where α, δ correspond to the Tycho catalog [6].The observing epoch for this catalog was J1991.25(equinox J2000). The rms uncertainties in thecoordinates are presented for each star, and are, onaverage, ±29 milliarcsecond (mas) and ±23 mas in αand δ, respectively.
We identified the measured stars with referencestars using the coordinates α, δ projected onto theplate (at the epoch of our catalog, J1975.11; i.e.,the observation date for reference plate No. 4261,equinox J2000). We obtained the tangential coordi-nates xref
tang, yreftang for 300 stars from the ACT located
in the region of sky encompassed by our catalog (here,the subscript ref denotes the reference catalog). Weidentified 174 stars with the Oricat catalog using theMIDAS software. Further, we derived the 10 plateconstants by obtaining a least-squares solution forthe 174 equations for the coordinate differences x −xref
tang and y − yreftang, then determined the tangential
123
124 VERESHCHAGIN, CHUPINA
Table 1. List of photographic plates from the Jena Observatory archives
Platenumber
Center coordinates, J2000 Date obtained Emulsion Filter Observer
α δ
96 5h31.0m −5◦25.8′ February 15/16, 1961 Astro-Special None Richter, Lochel
4261 5 32.4 5 27.0 February 8/9, 1975 Kodak IIa-0 None Borngen
4275 5 32.4 5 27.0 February 9/10, 1975 Kodak 103a-E RG1I Borngen
9110 5 31.7 5 24.5 June 16/17, 1996 ZU(2113) None Meusinger, Hogner
9160 5 31.7 5 24.5 December 12/13, 1996 ZU(2113) None Meusinger, Brunzendorf, Ludwig
9169 5 35.3 5 23.1 February 1/2, 1997 ZU(2113) GG13I Meusinger, Hogner
Table 2. Number of stars on the plates used in the reduction process
Plate number Numberof measured stars
Number of stars identifiedwith the reference plate
Number of starsafter removing edgezones of 7–10 mm
Number of referencestars
96 12 869 10 266 10 071 3315
4261 18 993 – 16 506 –
4275 16 256 12 635 12 266 3293
9110 19 213 12 414 16 977 3122
9160 12 370 8569 11 286 2767
9169 16 777 9401 9047 2358
coordinates for all stars in our catalog. These tan-gential coordinates were transformed into equatorialcoordinates using the projection of the plate onto asphere. In this way, we found α, δ at epoch J1975.11for equinox J2000. The mean rms uncertainty is3−4 mas.
Table 3. Plates for which corrections of the differenceswere made to take into account the brightness equation
Platenumber
Correcteddifference
Coordi-nates
Magnituderange
dxi dyi
96 Yes Yes x, y ≤ 17m
4275 Yes – x, y ≥ 18m
9110 Yes – x, y ≤ 18m
9160 Yes Yes x ≤ 17m
9160 Yes Yes y ≤ 16m
4. PROPER MOTIONS
To determine the proper motions, we obtained dif-ferential differences in the rectangular coordinates ofthe stars for each of the plates via comparison with areference plate. Plate No. 4261 was chosen as the ref-erence plate for two reasons: it has the largest overlapwith the other plate areas, and its observing epochis approximately in the center of the interval coveredby the observations. We transformed the measuredcoordinates of star i (xi, yi) to the system of thereference plate (xref
i , yrefi ) using standard procedures
in the MIDAS package. The numbers of measuredstars and of stars identified with stars on the referenceplate are given in Table 2. Analysis of the differentialcoordinate differences dxi = xi − xref
i and dyi = yi −yref
i indicated that these differed substantially from themean values at the plate edges. For this reason, starswithin 7−10 mm of the plate edges were not included,leading to a small reduction in the number of stars(Table 2).
We finally obtained a fourth-order polynomial re-gression fit, equalizing the quality of the images andrefining the dependences for the diffrential coordinatedifferences [11]. It was shown in [12] that this order for
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PROPER MOTIONS OF STARS 125
0
14 16 18 20
B
20
–20
μ
y
, m
as/y
r
0
14 16 18 20
20
–20
μ
x
, m
as/y
r
Fig. 1. Magnitude dependence of the proper motions for stars that are open-cluster members.
the polynomial fit is quite sufficient for the Tautenburgplates. The plate constants were calculated usingweak reference stars (18.0m ≤ m4261 ≤ 19.5m) withsmall proper motions (dxi, dyi ≤ 0.0035 mm).
The components of the proper motions μx, μy
and their rms uncertainties were obtained via a linearleast-squares fit to xi and yi as a function of the ob-serving epoch. A minimum of three plates were usedfor this purpose. The mean measurement uncertaintywas 3.5 mas/yr for magnitudes 15m ≤ m4261 ≤ 19m
and 5.0 mas/yr outside this interval.The brightness equation between plates was man-
ifest through the magnitude dependence of dxi ordyi. This dependence reflects a real dependence ofthe proper motions on the magnitude (brighter starsare, on average, closer and so have higher μ values).Table 3 presents the number of plates for which we in-troduced small corrections to the residual differencesusing the linear expression relating the residual differ-ences and m4261, until this dependence disappearedentirely.
Finally, the brightness equation was determinedusing stars that were members of open clusters. Theproper motions of such stars should not exhibit anymagnitude dependence. We used four clusters locatedin this region: NGC 1981, NGC 1977, the Trapez-ium, and NGC 1980. For each of the clusters, weselected stars with maximum-likelihood probabilitiesof cluster membership P > 63%. Corrections for thebrightness equation were made over several itera-tions, recalculating the cluster-membership proba-bility of the stars derived in the previous step. The final
numbers of selected cluster members with B < 18.5m
were 57, 9, 128, and 24 for NGC 1981, NGC 1977,the Trapezium, and NGC 1980, respectively. Weadopted a linear form for the brightness equation.The final uncertainties in the brightness equation inmas/yr were
ME(μx) = 14.89 − 0.747B,
ME(μy) = 6.10 − 0.313B.
Figure 1 shows the dependence of the proper mo-tions of cluster members on their magnitudes. Thisdisplays the brightness equation, which we took intoaccount in the Oricat catalog using the followingformulas.
5. COMPARISON OF CATALOGS
The proper motions in [4] and [5] have rms un-certainties of 0.54 mas/yr and 0.69 mas/yr, respec-tively; as we can see in Fig. 2, our data are in verygood agreement with these values. Table 4 presentsthe number of common stars, the mean differencesbetween the proper motions from various catalogs(“c”) and our Oricat catalog (“o”), as well as theirdispersions. Table 4 also gives the correspondingvalues for the ACT catalog, although its accuracy islower than the other catalogs considered. The formaldifferences for the HIP catalog [6] are also presented,although the disagreement between the HIP and ourown proper motions are small. This is due to thefact that we are dealing here with a small numberof bright (probably nearby) stars with approximately
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126 VERESHCHAGIN, CHUPINA
–40
–40 –20 0 20 40 –40 –20 0 20 40
–20
0
20
40
–40
–20
0
20
40
–40
–20
0
20
40
–40
–20
0
20
40
μ
α
(McN
amar
a et
al.
) m
as/y
r
μ
α
(Tia
n e
t al
.) m
as/y
r
μ
α
(AC
T)
mas
/yr
μ
α
(HIP
) m
as/y
r
μ
α
(Orical) mas/yr
μ
δ
(Orical) mas/yr
μ
δ
(McN
amar
a et
al.
) m
as/y
r
μ
δ
(Tia
n e
t al
.) m
as/y
r
μ
δ
(AC
T)
mas
/yr
μ
δ
(HIP
) m
as/y
r
Fig. 2. Comparison of proper motions from different catalogs. The points in the lower two panels show HIP data [6] and thehollow squares revised HIP data [7].
equal proper motions, whose differences are due pri-
marily to measurement errors. We can also see in
Fig. 2 that the data for the old and new versions of
the HIP catalog [6] and [7] are essentially the same.
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PROPER MOTIONS OF STARS 127
Table 4. Mean differences and corresponding dispersions of the proper motions between various catalogs (“c”) and theOricat catalog (“o”)
Catalog n 〈Δμx〉c-o 〈Δμy〉c-o σ〈Δμx〉c-o σ〈Δμy〉c-o
Tian et al. [4] 258 −3.00 −1.93 0.27 0.20
McNamara et al. [5] 560 −1.76 0.05 0.23 0.16
ACT [10] 174 5.08 −1.13 0.62 0.50
HIP [6] 22 3.99 −2.83 1.48 1.32
Table 5. Mean differences and corresponding dispersions of the proper motions between the PPMX catalog (“p”) andother catalogs (“c”)
Catalog n 〈Δμα〉p-c 〈Δμδ〉p-c σ〈Δμα〉p-c σ〈Δμδ〉p-c
Oricat 1050 0.47 0.90 11.23 12.40
Tian et al. [4] 73 −0.04 0.99 8.69 5.83
McNamara et al. [5] 211 1.22 −1.38 5.77 5.96
The PPMX catalog [16] contains astrometric andphotometric data for 18 million stars. The limitingmagnitude is RU = 15.2m in the GSC photometricsystem [17]. The accuracy in the proper motions is2 mas/yr for 66% of the stars and 10 mas/yr forthe remaining stars. The photometric data are takenfrom the ASCC-2.5 [18] and 2MASS [19] catalogs.PPMX consists of three parts: the first part (a) iscomplete to RU = 12.5m, the second part (b) con-tains high-accuracy μ values, and the third part (c)includes other stars from GSC 1.2 detected in the2MASS survey. We used part (a).
To cross-identify the stars in the Oricat andPPMX catalogs, we compiled a sample of stars fromthe latter whose coordinates lie in the part of thesky encompassed by Oricat (see Section 1). Recallthat Oricat contains 13 577 stars, while PPMX has7318 stars in the corresponding region. We carriedout the identifications automatically by comparing thepositions of the stars, as well as their R magnitudesin Oricat and RU magnitudes in PPMX. Stars weretaken to be identified if the difference in their positionsin the two catalogs did not exceed 1.5′′ and the dif-ference in their magnitudes was |RU − R| < 0.35m.This yielded cross-identifications for 1050 stars.
Figure 3 shows a comparison of the proper mo-tions for stars from the PPMX [16] and Oricat cat-alogs, as well as common stars from the catalogs ofTian et al. [4] (73 stars) and McNamara et al. [5](211 stars). The dispersions of these dependences aregiven in Table 5.
6. REDUCTION OF PROPER MOTIONS
TO THE HIPPARCOS SYSTEM
The considered area is located at low Galacticlatitudes in the Galactic zone of avoidance. Thus,it is possible to reduce the proper motions to aninertial system only by comparing them with propermotions in the HIP system [6]. Since there are only22 stars common to our catalog and to HIP [6], andthese are measured with large uncertainties (they arebright stars), we used stars from the AstrographicCatalog/Tycho (ACT) family of catalogs.
We compiled a master catalog of the proper mo-tions of 2156 stars brighter than 12m in a region5◦ in radius centered on the Trapezium cluster. Weused stars from the HIP [6] (336 stars), Tycho [6](2066 stars), ACT [10] (1931 stars), TRC [13] (1894stars), PPM [14] (793 stars), and CMC 11 [15] (1689stars) catalogs. The proper motions from the lastfive catalogs were reduced to the HIP system via acomparison with the HIP data [6], after which theywere averaged with corresponding weights. The Or-icat catalog contains 174 stars from this master cat-alog. The difference between the Oricat proper mo-tions (corrected for the brightness equation presentedabove) and the proper motions in the master catalogin the HIP system yields the following correction forreduction to the HIP system:
〈Δμx〉 = 〈μOrix − μHIP
x 〉= −9.41 ± 0.45 mas/yr,
〈Δμy〉 = 〈μOriy − μHIP
y 〉= −0.73 ± 0.37 mas/yr.
ASTRONOMY REPORTS Vol. 55 No. 2 2011
128 VERESHCHAGIN, CHUPINA
–20
–20
μ
α
(McN
amar
a et
al.
) m
as/y
r
μ
α
(Tia
n e
t al
.) m
as/y
r
μ
α
(Ori
cat)
mas
/yr
0 20 –20 0 20
0
20
–20
0
20
–20
0
20
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δ
(McN
amar
a et
al.
) m
as/y
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μ
δ
(Tia
n e
t al
.) m
as/y
r
μ
δ
(Ori
cat)
mas
/yr
μ
δ
(PPMX) mas/yr
μ
α
(PPMX) mas/yr
Fig. 3. Comparison of the PPMX proper motions with those of other catalogs: Oricat (upper plots), Tian et al. [4] (middleplots), and McNamara et al. [5] (lower plots).
7. MAGNITUDESThe filter–emulsion combination (Table 1) for
plate No. 9169 yields a broadband B magnitude,while that for plane No. 4275 yields R. The cal-ibration for the B magnitudes in the range 13m ≤B ≤ 16m was conducted using common stars fromthe photoelectric photometric catalog [20] (Fig. 4).For weak stars with 16m ≤ B ≤ 17m, the calibrationwas constructed in two steps. First, the relationshipbetween the photographic magnitudes mpg from thecatalog of Parenago [3] and m9169 was obtained:
mpg = −48.288 + 6.080m9169 − 0.142m29169 (1)
and then the relationB = −1.3394 + 1.0937mpg (2)
based on stars common to the catalogs [20] and [3].Substituting (1) into (2) yields the calibration relationB = f(m9169) for weak stars, which is also shown inFig. 4. This relation was used to determine the Bmagnitudes for 8710 stars from our catalog. The rmsuncertainty for our derived B magnitudes is 0.03m.
We constructed the R calibration using commonstars from the USNO-A2.0 [21] catalog, whose Rmagnitudes were determined from the Palomar SkySurvey plates, with a tie to CCD images:
R = −3454.193 + 1062.235m4275 (3)
− 130.460m24275 + 8.000m3
4275
− 0.244m44275 + 0.003m5
4275.
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PROPER MOTIONS OF STARS 129
The dependence RUSNO−m4275 is shown in Fig. 5.We used this relation to find the R magnitudes for11 642 stars from our catalog. The rms uncertaintyin the resulting R magnitudes is 0.25m. Figure 5shows that the USNO-A2.0 catalog is fully suitablefor calibration purposes to derive these magnitudes.
8. THE CATALOG
The Oricat catalog contains data for 13 577 starsin the region of the Orion Nebula. It is accessible viaCDS. The coordinates of the center are α = 83.45◦,δ = −5.20◦ (J2000), and the region is 3◦ × 3◦ in size.The catalog includes the following information:
num—the star number in the Oricat catalog (starswe have measured are numbered from 1 to 16 999;stars with numbers ≥17 000 were added from thecatalog of Parenago [3]);
num_par—the star number in the catalog of Pare-nago [3];
α2000, δ2000, σα, σδ—equatorial coordinates forepoch J2000 and their rms uncertainties (we deter-mined the coordinates of measured stars using thestandard procedure using reference stars from theACT catalog, see Section 3); the coordinates for starsfrom the catalog of Parenago [3] were translated toJ2000 taking into account only precession);
μx, μy , σμx , σμy —our measured proper motions(see Sections 4 and 6) and their rms uncertainties (inmas/yr);
μMNx , μMN
y , σMNμx
, σMNμy
—proper motions and theirrms uncertainties from [5];
μTx, μT
y , σTμx
, σTμy
—proper motions and their rmsuncertainties from [4];
μACTx , μACT
y , σACTμx
, σACTμy
—proper motions andtheir rms uncertainties from the ACT catalog;
μHx , μH
y , σHμx
, σHμy
—proper motions and their rmsuncertainties from the HIP catalog [6];
μHrevx , μHrev
y , σHrevμx
, σHrevμy
—proper motions and theirrms uncertainties from the revised HIP catalog [7];
μPPMXx , μPPMX
y , σPPMXμx
, σPPMXμy
—proper motionsand their rms uncertainties from the PPMX catalog;
B, R—our derived magnitudes;sp_par—spectra from [3];sp_Wal—МК spectra from [20].Figure 6 shows the distribution of stars from our
catalog in equatorial coordinates. The density of starsin the vicinity of the Great Nebula in Orion is low,and has a clumpy structure, due to the properties ofthe distribution of absorbing material. The positionsof the clusters NGC 1981, NGC 1977, OMC-2, theTrapezium, and NGC 1980 are shown. These clusters
8
14 16 18 20 22
m
9169
10
12
14
16
18
BFig. 4. Dependence of B on the instrumental magnitudem4275 (solid curve). The bold section of the curve wasdetermined using the mpg data from [3]. The circlesdenote points added to obtain the best fit.
8
1412 16 18 20
m
4275
6
10
12
14
16
18
R
Fig. 5. Dependence of R on the instrumental magnitudem9169. The solid curve shows a best-fit approximationcorresponding to formula (3).
are located in the region of Orion’s Sword. As canbe seen in Fig. 6, our catalog does not provide richmaterial for searches for new cluster members, but wehave successfully applied it, supplemented with addi-
ASTRONOMY REPORTS Vol. 55 No. 2 2011
130 VERESHCHAGIN, CHUPINA
–7.0
82.0 82.5 83.0 83.5 84.0 84.5 85.°0
α
–6.5
–6.0
–5.5
–5.0
–4.5
–4.°0
1
2
3
4
5
δ
Fig. 6. Distribution of Oricat stars in equatorial coordinates. The pluses indicate the positions of (1) NGC 1981, (2) NGC 1977,(3) OMC-2, (4) the Trapezium, and (5) NGC 1980. The center of the Great Nebula in Orion (M42) is near the point 4(α = 5h32m49s and δ = −05◦25′00′′) and has dimensions 85′ × 60′.
tional data, to study features in the spatial structureand kinematics of the region of Orion’s Sword [22].
9. CONCLUSION
We have used six photographic plates from theTautenburg astrograph to derive proper motions for12 740 stars in the vicinity of the Great Nebula inOrion. The rms uncertainties in the proper motionsare 3.4 mas/yr for magnitudes 6m ≤ B ≤ 10m and5.0 mas/yr outside this interval. We determined the Band R magnitudes of the catalog stars using calibra-tions we derived. We have found μ values for 784 newstars in the region considered in the previous mostmassive determination of proper motions, carried outby Parenago [3], and have supplemented our catalogwith the data of [3].
The Oricat catalog provides a basis for compilingmaster catalogs of astrometric parameters, and can
be used to study features of the structure and kine-matics of the stellar population in the vicinity of theGreat Nebula in Orion. As a continuation of thispaper, we have compiled a master catalog of stars inthe region of Orion’s Sword, which was used as abasis for studies of the spatial and kinematic structureof this region [22].
ACKNOWLEDGMENTS
We thank E. Schilbach, N.V. Kharchenko, andR.D. Scholz for numerous discussions of these re-sults, and J. Brunzendorf and H. Meusinger for helpin organizing the work at Jena Observatory.
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Translated by D. Gabuzda
ASTRONOMY REPORTS Vol. 55 No. 2 2011
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