EDP Sciences
Open Access
Issue
A&A
Volume 616, August 2018
Article Number L15
Number of page(s) 4
Section Letters to the Editor
DOI https://doi.org/10.1051/0004-6361/201833407
Published online 24 August 2018

© ESO 2018

Licence Creative Commons
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

The Milky Way has been thought to be a spiral galaxy since as far back as the 1850s (Alexander 1852), but it is extremely difficult to observe its spiral structure due to our edge-on view from its interior and limited by copious dust extinction. It was not until the 1950s that researchers started to use high-mass stars (OB stars) and H II regions to trace spiral arm segments in the solar neighborhood (Morgan et al. 1952, 1953). Such work, augmented by radio wavelength data, was extended to cover most of the Galaxy by Georgelin & Georgelin (1976) and Russeil (2003). In the last decade, Very Long Baseline Interferometry (VLBI) at centimeter wavelengths has provided hundreds of parallaxes to very young, massive stars with accuracies often as good as ±0.01 mas (Xu et al. 2006, 2016; Honma et al. 2007; Reid et al. 2014), substantially increasing our knowledge of the spiral arms in our Galaxy. Recent VLBI parallaxes, however, have mostly been limited to stars visible from the northern hemisphere, leaving the fourth Galactic quadrant largely unexplored.

The Gaia satellite (Gaia Collaboration 2016), launched in 2013, will ultimately achieve parallax accuracy comparable to that of VLBI for approximately 109 stars, although the current release (Gaia Collaboration 2018) is limited by systematic error of up to <0.1 mas, depending on celestial positions, magnitudes, and colors (Lindegren et al. 2018). Still, even with this limitation, distances to many OB stars within a couple of kiloparsec from the Sun are now available. We note that even when Gaia reaches its target accuracy, direct measurement of spiral structure will be limited by dust extinction in the plane of the inner Galaxy to stars typically within a few kiloparsec.

2. OB star sample

Owing to their short main sequence lifetimes, high-mass stars are generally located near their birth places, and consequently can trace Galactic spiral structure. However, due to the absence of effective temperatures (Gaia Collaboration 2018) for bright stars (>10 000 K) in Gaia DR2, we are unable to identify OB stars straightforwardly. In order to identify OB stars, we started with a sample of them from the Catalog of Galactic OB Stars (Reed 2003), which gives coordinates accurate to ≈1″, along with stellar classifications and Heliocentric radial velocities from SIMBAD1. We then cross-matched these stars with the Gaia DR2 catalog using a match radius of 1″. After having eliminated those with multiple matches, 5772 O-B2 stars remain (see Table 1). Our analysis starts with this sample.

Table 1.

Parallaxes and proper motions of OB stars.

Following the recommendation of the Gaia team (Arenou et al. 2018), we have not corrected the parallaxes of the individual OB stars in our sample by the average systematic bias of 0.029 mas on the parallax zero-point. Such a correction would, in any case, be very small for the nearby OB stars that we examine here. Table 1 lists coordinates, parallaxes, proper motions, radial velocities, and classifications of these OB stars. We believe this is the largest sample of O–B2 stars with parallaxes and proper motions to date.

3. Spiral structure

In this section, we investigate the spiral structure of the Milky Way within ≈3 kpc as revealed by OB stars and compare it with that determined from maser parallaxes associated with extremely young (<105 yr) high-mass star forming regions, whose parallaxes have been measured accurately with VLBI. Most of the stars in the sample have parallax uncertainties larger than ±20%, which can lead to distance uncertainty comparable to the spacing between spiral arms. Therefore, only stars with parallax accuracies better than ±10% are adopted, yielding a reduced sample of 2800 sources. We plot the locations of these stars in Fig.1, along with stars with VLBI parallaxes from Reid et al. (2014) and Xu et al. (2016), which extend much further into the Galaxy.

thumbnail Fig. 1.

Locations of Gaia DR2 OB (O–B2) stars (red circles) and masers (triangles) (Reid et al. 2014; Xu et al. 2016). The formal parallax uncertainties of the OB stars shown here are better than 10%, but do not include a possible ±0.1 mas systematic error. The Galactic center (red star) is at (0,0) and the Sun (Sun symbol) is at (0, 8.31). The solid and dashed curved lines denote the center and ±1σ widths of spiral arm models based on VBLI maser parallaxes. The Perseus arm (black), the Local arm (blue), and Sagittarius arm (green) are shown. Straight dashed lines indicate Galactic longitude.

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As expected, most of the Gaia OB stars in our reduced sample gather around the Sun within a radius of about 3 kpc, as distant OB stars have been rejected owing to their large distance uncertainties and/or extinctions. The Gaia data generally confirm the locations of spiral arms from VLBI in the Perseus, Local, and Sagittarius arms. Between Galactic longitude 240° and 360°, they can be used to extend our knowledge of the latter two arms.

As is also apparent in Fig. 1, OB stars presumably at nearly the same distance are spread out along the line of sight, suggesting that the 10% threshold for parallax uncertainties is insufficient at distances beyond about 1.4 kpc. In addition, a substantial number of OB stars appear to fall between spiral arms. Since this is rarely the case for the maser sample of very young and massive stars, it suggests that allowing stars as late as type B2 into the sample is blurring spiral structure. These stars can live long enough to orbit roughly half the way around the Galaxy and may have left the spiral arms.

In order to obtain an OB star sample that is truly capable of tracing spiral arms, we start over from our initial sample of 5772 stars, abandoning the 10% uncertainty constraint and removing B-type stars. For the remaining O-type stars, we incorporate the systematic error 0.1 mas on parallaxes recommended by Lindegren et al. (2018) and require absolute distance uncertainty <0.2 kpc and peculiar motions <20 km s−1 (the calculations of peculiar motions are given in the appendix). This leaves a sample of 59 O-type stars. These stars of O9.7V or earlier should have main sequence lifetimes of <8 Myr (Weidner & Vink 2010), and with peculiar motions <20 km s−1, they should not have moved more than 0.2 kpc from their birthplaces. Because the widths of the spiral arms neighboring the Sun are ≈0.3 kpc (Reid et al. 2014) and lines of sight usually are not perpendicular to spiral arms, these stars should still be in their birth spiral arms.

In Fig. 2a, we display the distribution of those 59 O-type stars. Most of them are consistent with being in the Local and Sagittarius spiral arms, as traced by masers. Interestingly, about 10 of the stars appear to be between these two arms. This observation should be reliable as it comes from a very conservative treatment of the Gaia DR2 parallaxes.

thumbnail Fig. 2.

Locations of only the identified O-type stars in Gaia DR2 (red circles): panel a) with ±0.1 mas systematic errors added in quadrature to the formal DR2 errors; only stars with corresponding absolute distance uncertainties <0.2 kpc are shown; Panel b) with ±0.05 mas systematic errors added in quadrature to the formal DR2 errors; only stars with corresponding absolute distance uncertainties <0.2 kpc are shown. See Fig. 1 caption for other details, but note that the 10% error threshold in that figure is not used here.

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If we relax the assumed DR2 systematic parallax uncertainty from ±0.1 to ±0.05 mas and loosen the absolute distance error constraint from ±0.2 kpc to ±0.3 kpc, the sample increases to 241 stars and is displayed in Fig. 2b. Significant linear arrangements that point toward the Sun (“fingers of God”) are now more apparent, suggesting that caution should be exercised when gleaning information about spiral structure from this sample. However, some interesting features, which are not strongly dependent on the distance uncertainty, can be seen.

The abundance of stars between about 240° and 270° longitude at distances >1 kpc suggest that the Local arm continues into the fourth quadrant and bends somewhat inward toward the Galactic center. Similarly, the stars between 300° and 320° longitude at distances >1 kpc extend the Sagittarius arm into the fourth quadrant with about the same pitch angle as that determined from the maser parallaxes. Finally, a small cluster of stars centered near longitude ≈290° at a distance of ≈2 kpc appear to extend upward from the Sagittarius arm at a fairly high angle, as one might expect for a Galactic spur. This should not be surprising as the maser parallaxes have already shown some spurs; for example, the Aquila spur (Cohen et al. 1980; Dame et al. 1986).

4. Conclusions

VLBI parallaxes have traced the Milky Way’s spiral structure within about 10 kpc of the Sun in Galactic quadrants 1, 2, and 3. The Gaia DR2 data supplement the maser VLBI measurements and extend the map of the Local and Sagittarius arms into quadrant 4. They also suggest an additional spur-like feature between those arms.


Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant Numbers: 11673066, U1431227, 11673051, 11703065, 11573054, 11503042), the Natural Science Foundation of Shanghai under grant 15ZR1446900, and the 100 Talents Project of the Chinese Academy of Sciences. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement.

References

Appendix A

The calculation of the peculiar motions of O stars

With distances, proper potions, and radial velocities, one has full three-dimensional (3D) velocity information in a heliocentric reference frame. Adding the Sun’s full motion with respect to a reference frame at rest at the Galactic center transfers the O-star motions to that frame. Then removing a model of pure circular motion gives Galactic peculiar motions, expressed by (Us, Vs, Ws), which are velocity components toward the Galactic center (GC), in the direction of Galactic rotation, and toward the north Galactic Pole at the location of each star.

We estimate peculiar motions of the O stars following Reid et al. (2009), using updated Galactic parameters of 241 km s−1 for the Galactic rotation speed, Θ0, at a distance of 8.31 kpc to the GC, R0, and solar motion parameters of U = 10.5 km s−1, V = 14.4 km s−1, and W = 8.9 km s−1 from Reid et al. (2014). We used the “Universal” rotation curve model, which produces a relatively flat rotation curve between 5 and 15 kpc.

All Tables

Table 1.

Parallaxes and proper motions of OB stars.

All Figures

thumbnail Fig. 1.

Locations of Gaia DR2 OB (O–B2) stars (red circles) and masers (triangles) (Reid et al. 2014; Xu et al. 2016). The formal parallax uncertainties of the OB stars shown here are better than 10%, but do not include a possible ±0.1 mas systematic error. The Galactic center (red star) is at (0,0) and the Sun (Sun symbol) is at (0, 8.31). The solid and dashed curved lines denote the center and ±1σ widths of spiral arm models based on VBLI maser parallaxes. The Perseus arm (black), the Local arm (blue), and Sagittarius arm (green) are shown. Straight dashed lines indicate Galactic longitude.

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In the text
thumbnail Fig. 2.

Locations of only the identified O-type stars in Gaia DR2 (red circles): panel a) with ±0.1 mas systematic errors added in quadrature to the formal DR2 errors; only stars with corresponding absolute distance uncertainties <0.2 kpc are shown; Panel b) with ±0.05 mas systematic errors added in quadrature to the formal DR2 errors; only stars with corresponding absolute distance uncertainties <0.2 kpc are shown. See Fig. 1 caption for other details, but note that the 10% error threshold in that figure is not used here.

Open with DEXTER
In the text

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