A&A 374, 288-293 (2001)
DOI: 10.1051/0004-6361:20010689
V. Straizys1 - K. Cernis1 - S. Bartasiute1,2
1 - Institute of Theoretical Physics and Astronomy,
Gostauto 12, Vilnius 2600, Lithuania
2 - Vilnius University Observatory, Ciurlionio 29,
Vilnius 2009, Lithuania
Received 15 September 2000 / Accepted 17 April 2001
Abstract
Vilnius seven-color photometry has been obtained for 238 stars down to
13 mag in the area of the California Nebula in Perseus.
For nearly all of the stars, photometric spectral classes, luminosity
classes, absolute magnitudes, interstellar reddenings, extinctions and
distances are determined. The "extinction versus distance'' diagrams
give evidence for the presence of one dust layer at
160 pc
distance in the direction of the California Nebula and its
nearest surroundings and of two dust layers at distances of 160 pc and
300 pc north-west of the nebula, in the direction of the dark clouds
L1449 and L1456. The front layer contributes
extinction between 0.3 and 1.3 mag, and the second layer gives about 1
mag of additional extinction. It appears likely that the front dust
layer is the extension of the Taurus dark clouds. The second dust layer
probably belongs to the complex of dark clouds found in other areas
of the southern part of Perseus (in the directions of the open cluster
IC 348, the reflection nebula NGC 1333, etc.). Both
cloud complexes run more or less parallel to the Galactic plane.
Key words: methods: observational - techniques: photometric - stars: fundamental parameters - interstellar medium: dust, extinction - nebulae: individual: California Nebula (or NGC 1499)
The complex of dark clouds in the Taurus, Auriga, Perseus and Aries
constellations is one of the largest concentrations of gas and dust in
the Galaxy. It extends 25
parallel to the Galactic
equator, which is equal to 61 pc at the 130 pc distance or 84 pc at the
180 pc distance, these distances corresponding to its front edge in
different directions. The depth of the complex is probably of the same
order, i.e. 60-80 pc. In many parts of the clouds, stars illuminate the
dust, forming reflection nebulae (e.g. the Pleiades, the
NGC 1333 nebula, etc.). In some areas, the ultraviolet
radiation of hot luminous stars ionizes the gaseous component of the
clouds, forming emission nebulae (like the California Nebula).
Radio observations reveal a number of molecular clouds hidden in the
densest dust concentrations. According to Ungerechts & Thaddeus
(1987) the total mass of all clouds in the complex is about
.
That paper also gives a map of the complex with
the molecular clouds, dust clouds, nebulae and clusters identified.
In the complex, star forming processes are taking place: in its eastern section (Taurus and Auriga) mostly low-mass T Tauri type stars are observed, while in the western section of the complex (Perseus) both low- and high-mass stars are being formed (the Per OB2 association, the open cluster IC 348, etc.).
In the seventies, we started a program of investigation of this complex of interstellar dust clouds by means of photoelectric photometry of stars in the Vilnius seven-color system. The following areas have been investigated: the Taurus area containing the Lynds dark clouds L1538, L1528, L1521 and L1495 (Straizys & Meistas 1980; Straizys 1982; Straizys et al. 1982a, 1982b, 1985), the Taurus area containing the Lynds dark clouds L1551, L1546 and L1543 (Meistas & Straizys 1981), the area around the variable star RV Tau (Straizys & Meistas 1981), the Merope dark cloud in the Pleiades (Cernis 1987), the area around the reflection nebula NGC 1333 (Cernis 1990), the areas around the open cluster IC 348 and in the Per OB2 association (Cernis 1993), the area around the open clusters NGC 1750 and NGC 1758 (Straizys et al. 1992). In these publications the distance vs. extinction relationships, cloud distances, densities and, in some cases, the extinction laws of the dust clouds were investigated. We concluded that almost everywhere in the Taurus and Perseus sections of the complex, the extinction begins to increase sharply at 130-180 pc. In the Perseus section, evidence was found for the presence of a second dark cloud at 230-270 pc. A model of the spatial distribution of Taurus and Perseus objects was discussed by Cernis (1993).
The present paper extends the research of the Perseus clouds in the
direction of the California Nebula (NGC 1499) which is
closer to the Galactic equator than the areas investigated earlier. The
area is bounded by the following 2000.0 coordinates: RA from
to
and DEC from
+35
to +39
.
The northern part of this area is covered by
the dark cloud Khavtassi 257 (Khavtassi 1960). In
the Lynds (1962) catalog, several smaller dark clouds,
L1449, L1456 and L1459, are separated
in the area.
The area includes several condensations of the molecular cloud situated north of California. This molecular cloud was investigated by Elmegreen & Elmegreen (1978), Wouterloot & Habing (1985), Ungerer et al. (1985), Ungerechts & Thaddeus (1987) and Herbertz et al. (1991) in the OH and CO radio lines.
The California Nebula itself is an emission region which covers
an area of
square degrees near the northern boundary of the
Per OB2 association. The source of its ionization is the
runaway star
Per (O7.5 III, V=4.04 mag).
Photoelectric photometry of 238 stars in the area was performed in the
standard passbands of the Vilnius system (Table 1) in
1994-1996 with the 1 m telescope of the Institute of Theoretical
Physics and Astronomy (Vilnius, Lithuania) situated at the Maidanak
Observatory in Uzbekistan. In the area, almost all stars down to V =
10 mag and many stars down to V = 11 mag and even fainter magnitudes
were measured. In the dark clouds north of California we observed a
number of fainter stars down to 13 mag (but not all). The typical
accuracy of magnitudes and color indices is better than 0.01 mag
(rms error), but for the faintest stars the accuracy can be somewhat
lower, especially in the ultraviolet filters U and P for red or
reddened stars.
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Figure 1: The chart for the California Nebula area showing the boundaries of the investigated area and its division into three areas of different extinction. The coordinates are for 2000.0. |
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The extinction coefficients of the atmosphere for every moment of observation were determined by the Nikonov (1976) method adjusted to the Vilnius system by Zdanavicius (1975, 1996). Transformation equations from the instrumental to the standard system were determined from observations of about 20 stars in the Cygnus Standard Region (Zdanavicius & Cerniene 1985). Details on the instrumentation and reduction procedures are given by Bartasiute (1999) and Straizys et al. (2001, hereafter Paper I).
The catalog of V magnitudes and color indices in the standard
Vilnius system for the observed stars is published in Paper I.
For the determination of spectral classes and absolute magnitudes (we call this process the photometric quantification) two independent methods were used.
(1) Interstellar reddening-free diagrams QUPY, QPYV;
QUPY, QXYV; QUXY, QUPYV; QUPY, QXZS and QXZS, QXYZ calibrated in terms of MK spectral
classes and absolute magnitudes MV by Straizys et al.
(1982c). The reddening-free Q-parameters
are defined by the equation:
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(1) |
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(2) |
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(3) |
(2) The -method of matching 14 different reddening-free
Q-parameters of a program star to those of about 7000 standard stars
of various spectral and luminosity classes, metallicities and
peculiarity types. This method does not give quantitative values of
stellar parameters but selects some standard stars which have a set of
Q-parameters most similar to those of the program star. If the
value is sufficiently low (i.e. the Q-differences between
the program and the standard star are small), the parameters of the
closest standard star may be prescribed to the program star. For
photometry of Population I stars with the 1% accuracy,
is
usually of the order of
mag for B-A-F-G stars and
of
mag for K-M stars.
The two methods used for the determination of spectral classes and absolute magnitudes give compatible results: the spectral classes agree typically to within 1 decimal subclass, the luminosity classes are usually the same in both cases.
The color excesses EY-V were calculated as differences between the
observed Y-V and the intrinsic color indices (Y-V)0 taken from
Straizys (1992) for stars of various spectral and
luminosity classes. The distances of stars were derived as usual:
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(4) |
The results of photometric quantification, i.e. spectral types, absolute
magnitudes, color excesses EY-V, extinctions AV, distances rand the quantification accuracies ,
are given in Paper I. That
paper also contains a comparison of distances determined from Vilnius
photometry with those from the Hipparcos parallaxes up to 250 pc. The
standard deviation of 23 stars in common is
25 pc in distance and
0.45 mag in absolute magnitude. No systematic difference is found
between the photometric and trigonometric distances.
According to the general trend of extinction with distance, the area can
be divided into three smaller areas with boundaries shown in
Fig. 1. The largest of the three, Area I, has a moderate
surface density of background stars and embraces the California
Nebula with its nearest surroundings. Area II is located north-west of
the nebula and includes the dark clouds L1449 and
L1456. Area III with the dark cloud L1459 is
located north-east of the nebula. In Areas II and III there are many
spots exhibiting a low surface density of background stars, especially
on the blue prints of the Palomar atlas.
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Figure 2: Interstellar extinction AV plotted against distance r in parsecs for Area I. The vertical broken line shows the accepted distance of the dust cloud at 160 pc. The broken curve corresponds to the limiting magnitude V=11.0 and MV = +1.0; all stars to the right of the curve are either apparently fainter or absolutely brighter. |
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Figure 2 shows the plot of the extinction versus distance for
Area I. Here, up to 110 pc distance, all stars are either
unreddened or only slightly reddened. The first two really reddened
stars are observed at a distance of 110 pc. More reddened stars start
to appear at
140 pc, but only three of them exhibit extinctions
larger than 1.7 mag. The extinction of the remaining stars in this area
is lower than 1.3 mag. The stars in the distance range from 250 pc to
about 1 kpc form, in Fig. 2, a belt between the AV values
0.3 and 1.3 mag, with a mean of
0.8 mag. This value can be
considered as the mean extinction in the direction of the
California Nebula.
There is no doubt that many more stars with AV>1.3 mag should be present in Area I, but due to considerable extinction they are apparently fainter than our approximate magnitude limit at V=11 mag (see the limiting curve for V=11 and MV = +1.0 mag, corresponding to A0 V and K III stars which are among the most luminous stars in the area).
A sudden appearance of the reddened stars at 110 pc means that
somewhere beyond this distance the dust cloud begins.
Due to the distance determination errors, the stars marking the
front side of the cloud should be scattered within approximately
25%, which at a distance of 160 pc gives about 40 pc
(rms error). If the first two reddened stars appear at 110 pc
due to this scatter, the cloud may actually be situated as
far as 150 pc.
On the other hand, the estimation of the distance to the cloud may be
obtained from the observed distances of unreddened stars. In the direction
of the area, the stars with very low extinction (
mag) are
seen up to 210 pc. Subtracting the rms error of 40 pc, we obtain a
distance of 170 pc to the front edge of the cloud. The two estimates, 150
and 170 parsecs, are sufficiently close to each other, taking into account
the fact that the closest reddened stars and the farthest unreddened stars
can be occasionally found within the 3
error box (three
standard deviations). Consequently, we are safe in considering the front
edge of the cloud to be at
160 pc distance. The absolute error of
this distance probably cannot be larger than
20 pc, since in the
opposite case it would be difficult to explain the presence of both
the reddened stars at 110 pc and the unreddened stars at 210 pc.
In Area I we do not find evidence of a possible second
cloud at larger distances. Probably, the only cloud present is that at
160 pc. In this respect Area I seems to be very similar to
the Per OB2 association area which contacts the
California Nebula from the south (Cernis 1993).
A one-cloud structure is also found in some other areas in Taurus and
Perseus (for further discussion, see Sect. 4).
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Figure 3: Interstellar extinction AV plotted against distance r in parsecs for Area II. The two vertical broken lines show the front edges of the dust clouds at distances of 160 pc and 300 pc. The horizontal broken line corresponds to AV=1.7 mag, the maximum extinction of stars affected by the first dust layer. The broken curve near top right corresponds to the limiting magnitude V=13 and MV = +1.0. |
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However, in Area II all stars (except one), which are farther than
350 pc, are affected by the extinction
mag, and the stars
with r<300 pc and AV>1.7 mag are completely absent. Heavily
reddened stars start to appear only at 300 pc. Another region in the
graph of Area II with r>350 pc and AV<1.7 mag is also empty, while
in Fig. 2 the corresponding region is well populated. Both these features
of the plot in Fig. 3 support the presence of a second dust
cloud. It is not likely that such distribution of stars in the AV vs.
distance plot would have been caused by selection effects.
Consequently, somewhere between r1=250 pc (the minimum distance of
stars with AV > 1.5 mag) and r2=350 pc (the maximum distance of
stars with AV < 1.5 mag) the extinction jumps up by 1 mag.
The most natural explanation of this feature seems to be the presence in
Area II of the second dust cloud at
300 pc. At this distance its
25% error corresponds to about 75 pc, which is more than sufficient
to explain the observed difference in limiting distances of stars with
high and low extinction in Area II.
Almost all stars with AV>1.8 mag are apparently fainter than V=11mag; with brighter limiting magnitude (as in Area I) they would be undetectable. Their spectral types are A2-A3 V-III and K2-K5 III.
Area III is covered by the dust cloud L1459, and almost all of the stars observed in it are closer than the cloud and are therefore mostly unreddened. Probably, our limiting magnitude in this area was insufficient to reach the stars inside or behind the dust cloud. Only three stars with AV between 0.5 and 0.9 are found.
Thus, we have found firm evidence that nearly all the investigated
area is covered by a dust cloud at 160 pc distance. As it was
already noted in a previous section, this distance is close to that of the
dust clouds in the Taurus and Perseus areas investigated by us earlier
using the same method. The extinction versus distance diagram for Area I
is found to be very similar to that obtained for the Per OB2
association area (Cernis 1993), while the same diagram for
Area II is similar to those for the areas of the reflection nebula
NGC 1333 and the open cluster IC 348 (Cernis
1990, 1993). The distances of the dust layers are
compared in Table 2. Since the calibration of the
Vilnius system in terms of absolute magnitudes in References 1-6 was
based on the Hyades distance modulus of 3.2 mag, all the distances
determined in these six papers were multiplied by 1.05 to adjust them to
the new Hyades distance modulus of 3.3 mag. The table also
includes the data for two areas observed by other authors, namely, the
area with the dark cloud L1478 in Perseus (Ungerer et al.
1985) and the area with the dark cloud L1457
(MBM12) in Aries (Hobbs et al. 1986). Both
clouds probably belong to the same complex of dust and molecular
clouds.
Layers | ||||||
Area | l | b | I | II | RV | Ref. |
(deg) | (deg) | (pc) | (pc) | (kms-1) | ||
Taurus 1 area | 174 | -14 | 150 | - | +6.3 | 1 |
Taurus 2 area | 178 | -21 | 150 | - | +7.0 | 2 |
Merope cloud | 167 | -24 | 130 | - | +9.7 | 3 |
NGC 1333 area in Per | 158 | -20 | 170 | 230 | +6.8 | 4 |
NGC 1750/58 in Tau | 179 | -11 | 180 | - | +6.0 | 5 |
IC 348 area in Per | 160 | -18 | 170 | 270 | +8.5 | 6 |
Per OB2 area | 160 | -15 | 170 | - | +8.2 | 6 |
L1478 cloud in Per | 163 | -9 | 135 | 380 | -2.7 | 7 |
L1457 cloud in Ari | 159 | -34 | 65 | - | -1.2 | 8 |
California Nebula area | 160 | -12 | 160 | 300 | -5.0, | 9 |
+3.1 |
References: 1. Straizys & Meistas
(1980), 2. Meistas & Straizys
(1981), 3. Cernis (1987), 4. Cernis (1990), 5. Straizys et al. (1992), 6. Cernis (1993), 7. Ungerer et al. (1985), 8. Hobbs et al. (1986), 9. This paper. |
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Figure 4: The polar diagram "distance versus galactic latitude" for the Taurus, Perseus and Aries dust clouds. The dust clouds are shown by solid dots (the first layer) and open circles (the second layer). |
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The two dust layer structure in Perseus has also been detected by other
authors. Probably, the first indication of the two layers of the
CO molecules in the dark cloud north of the California
Nebula was found by Elmegreen & Elmegreen (1978). Later on,
Ungerer et al. (1985) investigated a dust concentration in the
L1478 molecular cloud located 3
east of the nebula. From
UBV photometry and spectral classification data they identified two dust
layers at distances of 135 pc and 380 pc. The presence of two dust layers
in Perseus was confirmed by Ungerechts & Thaddeus (1987) from
analysis of the available extinction investigations. They suspected that
the Taurus dark clouds at
140 pc distance extend into Perseus, while
the molecular and dust clouds associated with the IC 348 cluster,
NGC 1333 nebula and Per OB2 association are farther away.
They accepted a distance of 350 pc for the clouds associated with the
California Nebula, as well as for the chain of molecular clouds
extending from California towards Auriga. In some parts of these clouds,
double or triple lines of the CO molecule were observed, pointing to the
presence of two or more gas layers with different radial velocities. In
the dust clouds north of the California Nebula Herbertz et al.
(1991) found a complicated pattern of cloud fragments with
average velocities near -7, -3 and +3 kms-1. Wouterloot & Habing
(1985) have also found evidence for the two clouds from OH
radio line observations.
Further evidence for the two layers at different distances in the Per OB2 association area was found by Krelowski et al. (1996) from analysis of the shapes of extinction curves in the ultraviolet, the strength ratio of two diffuse interstellar bands at 578 and 580 nm, and the strengths of interstellar CN and CH molecular bands and sodium lines. They suspect that the two clouds have different absorbing properties. Sonnentrucker et al. (1999) have also found two clouds with different radial velocities observed in the 661 nm diffuse interstellar band. Kaczmarczyk (2000) recently identified two components of the interstellar C2 bands in the ultraviolet spectra of the star X Per, also corresponding to two gaseous layers with different radial velocities.
Cernis (1993) has proposed a model of distribution of the two dust layers, based on his finding that the distance of the Perseus clouds increases with the apparent approach to the Galactic equator. This means that the complex of dust clouds in the second layer may be more or less parallel to the Galactic plane.
The same tendency is confirmed by the present paper, as can be seen from the polar diagram shown in Fig. 4. On this diagram the data for the seven areas investigated by our team earlier are plotted together with the new data for the California area. Additionally, the areas of the dark clouds L1478 (Per) and L1457 (Ari) are also shown. The clouds of the second layer form an elongated complex which is situated 60-80 pc below the Galactic plane and is inclined by a small angle to the plane.
The clouds of the first layer concentrate in a formless group at
130-180 pc from the Sun and 20-60 pc below the Galactic plane. The
spread of these clouds in longitude is about 20 degrees. No correlation
is found between the longitude and the distance from the Galactic plane.
This dust layer may also be parallel to the plane. This is shown by
the position in Fig. 4 of the Aries cloud L1457
at
.
Its distance from the Sun,
65 pc, was estimated
by Hobbs et al. (1986, 1988) and Hearty et al.
(2000a). However, this distance is very uncertain and,
according to Hearty et al. (2000b), may be somewhat larger
(58-90 pc). Probably, the Aries cloud belongs to the same complex as
the clouds of the first layer in Taurus, Auriga and Perseus, although
its radial velocity is somewhat different (see Table 2).
It is known that the Sun is located about 10 pc above the Galactic
plane. In this case, all the distances of the clouds from the Galactic
plane should be reduced by this amount. According to Cernis
(1993) and de Zeeuw at al. (1999), the Per OB2
association starts at 33816 pc and 318
27 pc distances,
respectively. Thus, the distances of the association and the dust
clouds north of California coincide within the distance
determination errors. The distance of the second layer determined in
the present paper, 300 pc, is smaller than 350 pc, the distance accepted
by Ungerechts & Thaddeus (1987). This means that their
estimation of the mass of the Perseus clouds may be slightly
overestimated.
Future investigations should help show where the clouds of both
layers start and end. The second layer may either end
abruptly at some distance from the Sun or may reach the Galactic plane.
When approaching the Sun, both layers may merge at 150 pc
distance and at galactic latitude between -20
and -30
.
Therefore, additional investigations of the cloud complex at lower and
higher latitudes are desired. On the other hand, we need more exact
distances to the Aries dark clouds for the estimation of their relation to
the Taurus-Auriga-Perseus complex. The investigation of the Aries region
by our group is in progress. It is also important to verify if
interstellar lines in the spectra of the heavily reddened early-type
stars of Area II exhibit two components corresponding to the clouds with
different radial velocities.