Getting GD-1 Data¶
From the Price-Whelan and Bonaca paper, we will try to reproduce Figure 1, which includes this representation of stars likely to belong to GD-1:
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Along the axis of right ascension (\(\phi_1\)) the figure extends from -100 to 20 degrees.
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+Along the axis of right ascension (\(\phi_1\)) the figure extends from -100 to 20 degrees.
Along the axis of declination (\(\phi_2\)) the figure extends from about -8 to 4 degrees.
Ideally, we would select all stars from this rectangle, but there are more than 10 million of them, so
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diff --git a/03_motion.html b/03_motion.html
index f96a8ed..da18163 100644
--- a/03_motion.html
+++ b/03_motion.html
@@ -904,7 +904,7 @@ The coordinates in
Due to the nature of tidal streams, we expect the proper motion for most stars to be along the axis of the stream; that is, we expect motion in the direction of
phi2to be near 0.
@@ -915,7 +915,7 @@ The coordinates in What is the primary scientific result of this work?
What story is this figure telling?
Plot proper motion¶
Now we are ready to replicate one of the panels in Figure 1 of the Price-Whelan and Bonaca paper, the one that shows the components of proper motion as a scatter plot:
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In this figure, the shaded area is a high-density region of stars with the proper motion we expect for stars in GD-1.
Selecting the centerline¶
As we can see in the following figure, many stars in GD-1 are less than 1 degree of declination from the line phi2=0.
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If we select stars near this line, they are more likely to be in GD-1.
We’ll start by selecting the phi2 column from the DataFrame:
@@ -1077,7 +1077,7 @@ Name: phi2, dtype: bool
Filtering based on proper motion¶
The next step is to select stars in the “overdense” region of proper motion, which are candidates to be in GD-1.
In the original paper, Price-Whelan and Bonaca used a polygon to cover this region, as shown in this figure.
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We’ll use a simple rectangle for now, but in a later lesson we’ll see how to select a polygonal region as well.
Here are bounds on proper motion we chose by eye,
diff --git a/04_select.html b/04_select.html
index 389b57e..470db48 100644
--- a/04_select.html
+++ b/04_select.html
@@ -902,8 +902,8 @@ Results: None
We’re starting to see GD-1 more clearly.
We can compare this figure with one of these panels in Figure 1 from the original paper:
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The top panel shows stars selected based on proper motion only, so it is comparable to our figure (although notice that it covers a wider region).
In the next lesson, we will use photometry data from Pan-STARRS to do a second round of filtering, and see if we can replicate the bottom panel.
We’ll also learn how to add annotations like the ones in the figure from the paper, and customize the style of the figure to present the results clearly and compellingly.
diff --git a/05_join.html b/05_join.html
index 2a7c786..da2abb6 100644
--- a/05_join.html
+++ b/05_join.html
@@ -309,7 +309,7 @@
This is the fifth in a series of notebooks related to astronomy data.
As a continuing example, we will replicate part of the analysis in a recent paper, “Off the beaten path: Gaia reveals GD-1 stars outside of the main stream” by Adrian M. Price-Whelan and Ana Bonaca.
Picking up where we left off, the next step in the analysis is to select candidate stars based on photometry. The following figure from the paper is a color-magnitude diagram for the stars selected based on proper motion:
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In red is a theoretical isochrone, showing where we expect the stars in GD-1 to fall based on the metallicity and age of their original globular cluster.
By selecting stars in the shaded area, we can further distinguish the main sequence of GD-1 from younger background stars.
diff --git a/06_photo.html b/06_photo.html
index 4d305be..2103430 100644
--- a/06_photo.html
+++ b/06_photo.html
@@ -325,7 +325,7 @@
As a continuing example, we will replicate part of the analysis in a recent paper, “Off the beaten path: Gaia reveals GD-1 stars outside of the main stream” by Adrian M. Price-Whelan and Ana Bonaca.
In the previous lesson we downloaded photometry data from Pan-STARRS, which is available from the same server we’ve been using to get Gaia data.
The next step in the analysis is to select candidate stars based on the photometry data. The following figure from the paper is a color-magnitude diagram for the stars selected based on proper motion:
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+
In red is a theoretical isochrone, showing where we expect the stars in GD-1 to fall based on the metallicity and age of their original globular cluster.
By selecting stars in the shaded area, we can further distinguish the main sequence of GD-1 from younger background stars.
@@ -393,7 +393,7 @@
Plotting photometry data¶
Now that we have photometry data from Pan-STARRS, we can replicate the color-magnitude diagram from the original paper:
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The y-axis shows the apparent magnitude of each source with the g filter.
The x-axis shows the difference in apparent magnitude between the g and i filters, which indicates color.
Stars with lower values of (g-i) are brighter in g-band than in i-band, compared to other stars, which means they are bluer.
diff --git a/07_plot.html b/07_plot.html
index 334724c..dacb57f 100644
--- a/07_plot.html
+++ b/07_plot.html
@@ -384,7 +384,8 @@
Not necessarily in that order.
Let’s start by reviewing Figure 1 from the original paper. We’ve seen the individual panels, but now let’s look at the whole thing, along with the caption:
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Exercise: Think about the following questions:
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+Exercise: Think about the following questions: